A portrait from Yannick Fornacciari's new book documenting trans people

For transgender men, injecting testosterone is a fundamental part of their transition. Bone mass, voice pitch, energy levels, temperament – the list of physical and psychological attributes it has the potential to alter is exhaustive. That testosterone ramps up the libido is common knowledge, but for some trans men it doesn't just heighten their sexuality, it shifts it altogether.

"Prior to my transition I definitely considered myself a lesbian for sure," says 33-year-old Will Krisanda. "I was primarily attracted to women but now it's completely shifted, and I'm finding myself more attracted to men. After about a year [on testosterone] I started to accept my sexuality as a bisexual. That took me by surprise because I've always been more comfortable with women. Testosterone is a powerful thing that's going into your body and it has some very interesting and permanent changes."

Though he identifies as bisexual, Krisanda is currently only interested in exploring romantic and physical relationships with men. Even if his friends offered to set him up on a date with a potentially well-matched female, he says he'd probably decline the offer. In less than a year, Krisansda had gone from exclusively dating women, to exclusively dating men. His experience isn't an uncommon one in the female-to-male (FTM) community.

Prior to his transition, filmmaker Jake Graf identified as a lesbian and had never had any inclination towards men, either sexually or romantically. From the age of 17 he worked in the recently closed lesbian nightclub Candy Bar and spent the next decade dating many girls. At the age of 28 he took a trip to New York and witnessed the city's trans scene, a journey which inspired him to begin his own transition. Like Krisanda, it took less than a year on testosterone for his sexual orientation to shift entirely.

"From when I was two or three years old all I knew was that I liked girls and that I was a boy," says Graf. "But within I would say about a year on testosterone I started looking at guys in the street thinking 'he's quite cute', which is obviously quite weird because I'd never ever looked at them before. It was weird and it was totally confusing. There's a lot of adjustment when you're transitioning and then add to that the fact that all of a sudden all I could think about was my newfound attraction to guys – it was a double head-fuck."

Throughout the first year of his transition Graf was with his long-term girlfriend. Though he found himself becoming attracted to men he still loved her and stayed with her, hoping the feelings would go away. It wasn't until she relocated abroad, effectively ending their relationship, that he began to explore his new desires.

"It wasn't just that I started fancying guys but still liked girls, it was literally that I didn't look at girls any more," he says. "Weirdly, the thought of having sex with a girl just left me really cold for a while. So I was going on the gay dating apps, going out to bars; it was kind of all-encompassing what with this massive sex-drive, because obviously your sex-drive ramps up through the roof anyway. It's insane, you just want to be having sex the whole time. It was just overwhelming."

Graf was recently told that his testosterone levels were quite high and after they were reduced he found that his sexual preferences eventually "levelled out". Though he still has a strong attraction to men, he is now much more open about with whom he has both romantic and physical relations.

"At the moment I'm sort of open," says Graf. "Now that the testosterone has calmed down a little bit I'm the old me, which was always into girls. Now I'm very much about – and I dislike the term 'bisexual' and I certainly would never say 'pansexual' – but I think now I'm just into people. I like strong charisma and cool people, as oppose to either or."

Johannes Fuß, a sex researcher at Hamburg University's Institute for Sex Research and Forensic Psychiatry, conducted a study with his research team into the shifting sexuality experienced by trans men and women during their orientation. Though it was clear that transition-related events such as surgical interventions and hormonal therapy did affect sexual behaviour, the study was unable to determine conclusively why exactly the self-reported changes occurred.

"I think some trans people are confused regarding their sexual orientation before transitioning," says Fuß. "Some encounter intense feelings of disgust regarding their own sexual characteristics, some envy people that have the sex characteristics that they are longing for and some feel ashamed when others see them naked. All this confusion sometimes seems to make it hard for trans people to express themselves sexually, and to be absolutely sure about their sexual orientation."

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According to Fuß, the onset of bodily changes as a result of surgery and hormones has the potential to end dysphoria and allow them to begin exploring their sexuality more freely. Dee began her transition ten months ago and, as a result of the hormones, already feels more comfortable and liberated sexually. Though she didn't consider her sexuality much prior to her transition – a result of not enjoying sex of any kind – since coming out as trans she has been able to experiment confidently and now identifies as pansexual.

"The sex is a lot more satisfying now, it's just a much more pleasurable experience," she says. "I feel like my body is more in tune with what I'm trying to experience. What it was before was just really complicated and not enjoyable.

"I don't know if it's so much that the hormones changed my tastes or if it's just the fact that being my authentic self has allowed me to open up to the possibilities of sleeping with different people. It's like now that that skeleton is out of the closet I'm allowed to be attracted to whomever I feel like. I think it just comes down to accepting who you are and that allows you to be a lot more open and accepting of other people."

While the physiological impact of taking testosterone and transitioning is likely contributing to these significant changes in sexual orientation, some who've had similar experiences feel there's also a mental and emotional element to it.

"Before I transitioned I considered myself a lesbian," says New York-based writer, Emmett Lundberg. "I was attracted to women and maybe I had a couple of crushes on guys but it was never anything substantial. But I consider myself queer now; I'm attracted to everyone. For me I'm sure it's some combination of the hormones and then you're feeling comfortable with yourself, so it's kind of like you're open to other possibilities. I certainly was much more aware and comfortable verbalising my feelings towards men after I started transitioning."

Like many other trans men, 34-year-old Tucker had no interest in having sex with men until after he'd started his transition. Similarly for him, it was overcoming his gender dysphoria which led him to explore his new-found attractions to men.

"I was very uncomfortable sexually until I started testosterone. I definitely would never have sex with men before testosterone, but I've had sex with plenty of men after. When you have sex with a man as a woman there's definitely the dynamic where you feel like a woman," says Tucker. "So I feel like if you're having gender issues, just the act of having sex with a man – and you not passing – definitely makes you feel like a woman, which is very uncomfortable for you. Whereas if you are passing it really changes the dynamic. It changes it drastically."

While the development of a more fluid sexuality is, according to Tucker, a common experience in the trans male community it's not one that is talked about much. But even for those who are aware that such a phenomenon can occur, it can still come as a surprise and requires adjusting to.

"I think it's much more common than is necessarily even talked about in the community," says Lundberg. "It definitely threw me off because I did know that trans guys transition and then became attracted to men but I guess I didn't really think about it in my situation. So it did catch me a little off guard but now I feel much more comfortable with it and I just feel really open to anyone."

Having previously come out as a lesbian and then as trans, coming to terms with his homosexual feelings and dealing with coming out again was a familiar experience, one he didn't dwell on too much. But even with the knowledge and experience of growing up trans, accepting same-sex attraction isn't necessarily an easy journey. Will Krisanda is now in a place where he's happy to explore relationships with other men, but getting there was a struggle.

"When I started my first year, I was very much in this mode of wanting to be seen as masculine as possible," says Krisanda. "It was really hard for me to admit that there was a time when I thought that it was a bad thing to be attracted to men as a male. I just didn't know how to explore that part of me with this new exterior. I think it was just over-compensating my lack of 100 percent presenting as male by just trying to be as masculine as possible and just saying no, being attracted to men makes me less male – which is totally not the case."

"So this attraction to men, especially in the last year to two years, has really been a learning process for me to try to accept myself now, as a man who is attracted to men. It's definitely been a challenge, but a good challenge."