In some gay (and straight) relationships this would not be a problem. In others of course it would. I think it is by far the best thing if you talk openly with your boyfriend about what happened, telling him how it made you feel and why it made you feel that way. You need to think carefully and very honestly about this. I have had 3 long term gay relationships and 2 of these were open and the present relationship is monogamous. But even in an open relationship I would have felt hurt by what your boyfriend did, for several reasons:
- This happened more or less in your presence and you were excluded from it.
- It was not discussed beforehand.
- Your feelings were not taken into account
Please don't just adopt my attitudes and opinions. Some people would not be bothered even though they might have taken into consideration my concerns. How do you REALLY feel about what happened? I have known men who were turned on by their boyfriend's sexual adventures and to me that is OK.
There is another dimension to this too. How do you see your relationship developing? How do you WANT it to develop. At the very least you need to discuss what the ground rules are for you two in the future. Respect is all important but also respect for each other's needs. If you cannot accept the need that he has for some freedom (if this is a real need for him), then you are left only with plan B – leave him before things get worse. Self respect is pretty important too (English understatement!)
When you discuss this with him, your aim should not be to guilt-trip him but simply to find a way forward. You may decide to love him and leave him but you should at least respect your mutual differences and can in time be friends, if not boyfriends. The last thing I would want is to impose my views on you, so none of the above is my advice: it's just stuff to think about.
I wish you both lots of luck whatever the outcome.
PS. I don't think alcohol is an excuse. Drunks say and do what they really want to say and do. It's sober people who conceal their true feelings. I am sure there will be times in the future when you both get drunk again.
PPS. Whether something happened or not is not really the issue here, is it?