The dilemma I’m in a sweet, monogamous relationship with my girlfriend. We’re in our early 40s. She’s beautiful and we have a happy sex life. But I’m also fascinated by her friend, who leads a libertine lifestyle. I keep thinking about threesomes and other kinky games. I love my girlfriend, but I find other women attractive, too.

I’m loyal and I’d never cheat, but my promiscuous imagination is hard to repress – it comes out in pillow talk and in jokes and innuendos. My girlfriend has a good sense of humour and says it’s just the nature of my sexuality, the same way it would be if I was gay. But it hurts her feelings and it’s coming between us. My old Catholic sensibility says it’s a sin and I should fight it. What do you think? Am I a male pig trying to have his cake and eat it? How do I pursue my happiness without hurting the woman I love?

Mariella replies Imaginatively, like the rest of us? In these literal times it’s often overlooked that some of the best sex you’ll ever have takes place in the space between your ears. Just because you fantasise about something doesn’t mean you have to do it – or there wouldn’t be a long-term union left intact. But if cerebral adventures aren’t enough, then prepare yourself to make some compromises.

I really don’t think your dilemma is the same as being gay. If you’re homosexual, personal choice doesn’t come into the equation, whereas what you’re describing here is a lifestyle option. You may not be a swine, but you definitely want to have your cake and eat it. A sense of humour on your girlfriend’s part may not be enough to facilitate your pleasures. In fact, showing interest in her freewheeling best friend is more likely to entirely wipe that smile off her “beautiful” face.

If you’re lusting for stimulation and your girlfriend isn’t, it’s time to reconsider for both your sakes

Before we get to the nitty-gritty, though, I’d like to congratulate you on bothering to ask. The season of inappropriate behaviour is upon us and there are plenty of people out there using alcohol and the festivities as an excuse for random acts of infidelity. It would barely raise an eyebrow if a Christmas party were to lead to one of your fantasies springing to life and, such is our dysfunctional relationship with booze, you’d find plenty who’d see that as perfectly understandable. “Knee-tremblers” and “bog snogs” reach their highest incidence levels as we embrace our most conspicuous religious festival with an orgy of less-elevated behaviour.

If it’s an excusable indulgence you’re after, the run-up to 25 December couldn’t offer better scope, but it won’t solve your longer-term dilemma. That’s why pausing to consider your potential actions and asking advice are both admirable steps. Could it be that you’re with the wrong girl? You make being part of a “sweet, monogamous relationship” sound like a personal compromise, so perhaps you need to consider whether she’s the one for you. Perhaps you’ve wound up with Samantha Bond’s Miss Moneypenny when Famke Janssen’s voracious Xenia Onatopp would be the better match.

There’s nothing wrong with monogamy, but if you’re lusting for further stimulation and your girlfriend doesn’t want to partner you down that road it’s time to reconsider for both your sakes. There are plenty of women around who’d enjoy a role in your sexual adventures. Whether they’d also provide you with a “sweet” and “happy” relationship is another matter.

Further steps toward realising your sexual desires will have repercussions. As a Catholic you’re programmed to feel guilty and easily let off the hook, but I don’t think you want to make deception your modus operandi. You also don’t want to become some sad old salacious pleasure seeker, never sated and always on the look out for further adventure.

There’s nothing wrong with your fantasies – they’re pretty pedestrian by today’s standards – but you need to be comfortable with your desires and with a willing playmate. Freud was adamant that individual sexual desire is at the heart of who we are, but often, instead of being celebrated as an imperative component of self-discovery, it’s relegated to hobby status. Only you can gauge how imperative it is to you to physically explore your erotic dreams, but the answer needs to inform your choice of partner.

We all have red lines in our relationships and monogamy isn’t always where the buck stops. Establishing individual boundaries is an important part of getting to know each other and learning to live together. Accepting that living out your fantasises requires a partner who wants to accompany you is the baseline here. What you shouldn’t do is betray your girlfriend, proposition her best friend and exonerate your choices by blaming them on unavoidable urges. There is no right or wrong here, it’s all about making a mature decision about what you value most. But “having it all” in the field of desire is the most delusional fantasy of all.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. Follow her on Twitter @mariellaf1