It sounds as if you’ve never discussed ground rules, says Annalisa Barbieri, and that you have different ideas of what is permissible

My girlfriend and I are in our mid to late 20s and have been together for four years. Before we met, we both had sexual experiences with members of the same sex. While I’ve never felt romantically attracted to a man, she has towards a woman. I might be bicurious; she is more bisexual.

Since we’ve been together, we’ve had one threesome (involving another woman). A handful of times, at parties, they made out in my presence, and with my consent. Otherwise, we have an exclusive relationship and are clear about never cheating.

Recently, she came home from a party (without me) and told me she had made out with a woman who is a close friend of hers. When she told me, I didn’t know what to feel: I warned her to be careful not to ruin her friendship. I knew that I didn’t love the idea.

Over the following weeks, my feelings became clearer. My girlfriend and I had a falling out; I was angry and felt cheated on. The fact that they were close friends meant I was put in the position of having to be OK with them “just hanging out”.

My girlfriend doesn’t have a lot of friends and it would be unreasonable to object to them seeing each other. This is hard for me, though, because it’s easy to think it will happen again – or that their friendship will evolve into something else. I told my girlfriend my fears; she understood, and guaranteed it would never happen again. She also told me she only kissed her because she thought I would be OK with it, and that the lines were blurred. It has been a month or so now since it happened. I don’t know how to handle it.

When you have an open relationship, one that allows other people in, it doesn’t mean you have to be OK with everything that happens. At least part of your struggle seems to involve you trying to quash your feelings. I notice that immediately after it happened you warned your girlfriend to be careful not to ruin her friendship with the other girl – not your relationship. Why was that? It seems that the elasticity of your relationship accommodates your girlfriend more than you.

Did you open up your relationship to include others from the beginning, or is it a recent thing? Are you doing it because you both want to – or because she does? It sounds as if you’ve never really discussed ground rules, and that you and your girlfriend have different ideas of what is permissible. That she willingly told you about kissing her friend is a good sign, but she has taken things into a different arena.

There is, as sex and relationship counsellor Murray Blacket (cosrt.org.uk) pointed out, “a difference between having a sexual relationship with your regular partner and bringing others into it by choice and agreement [what you had done in the past], and a situation when one of you splinters off independently to be with someone else”.

Blacket added: “I think you need to have the conversation about whether your relationship is polysexual – you have sex with others, either together or separately, usually just once or twice, but there is no relationship – or polyrelational – when you also form a sexual relationship with someone outside the dyad.”

The lines have indeed become blurred. Your girlfriend kissed a woman she already has a friendship with – so now that relationship has turned sexual. Whether this was just a bit of fun, or is likely to turn into something beyond that one kiss, is really the question; but the fact that several weeks later you still don’t feel comfortable with it can’t be ignored.

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I get the sense that you are trying to be cool about the situation. But whatever sort of relationship you have (exclusive, open, somewhere in between), it’s important, not to say liberating, to be able to tune into your feelings and admit what you want, and not to put up with things you’re not happy with.

Discuss this with your girlfriend; be brave, be honest. Draw up some ground rules, which could change over time if mutually agreed on; this is why ongoing communication is key. Only when it’s clear what is and isn’t permissible to you both will you give the other person a chance to show whether or not they are the partner for you.

• Send your problem to annalisa.barbieri@mac.com. Annalisa regrets she cannot enter into personal correspondence.

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