I've made a big mistake, need help.
Here's what happened. I've been in a very loving relationship for 5 years. We were both coming out of bad relationships and in retrospect, should have been single a little while before jumping into this one. I've always considered myself to be a very non-jealous boyfriend (I don't keep tabs on her when she goes out and don't freak out over male friends) and the possibility of an open relationship has been discussed, although never really decided on.
Anyway, on new years eve, I had to work while she went to a party. She began to flirt with a guy she's known for years and they made out. She told me about it the next day and I felt no jealousy, I found it kind of funny and we both laughed about it and got into a further discussion about what, if any, our limits are with other people. I told her that I wasn't jealous and that I couldn't see how what she did with someone else would affect how we felt about each other. I really felt this at the time.
A few nights later, she went out again with the same group of people and ended up going much further with the same guy (not sex, but about as close as you can get, hands were involved from both parties). She told me about it the next day and I told her it was fine. That night however, I couldn't sleep more than a couple of hours, I was haunted by visuals in my head of another man's hands on her body and her enjoying it. When she woke up I told her that I was having some issues with this after all.
Neither of us was angry at the other but it was an emotional morning. She told me that she never would have let it go any further if I hadn't given her permission. She swore to me that it is purely physical and that she has no emotional attachment to this guy whatsoever. I told her it was okay and that I would just have to get over it being that I did give her permission and she hadn't done anything wrong.
I spent the next two days depressed but letting it go. She came to me soon after and said that she is still very physically attracted to him and does want have sex with him. We got together when she was 19 (she's 24 now). I was 21 at the time. Neither of us has had many sexual partners. And I know she sometimes regrets not staying single for at least a few months after her last relationship to get some craziness out of her system.
I told her I had to think about whether it was okay with me if she had sex with him. She assured me that she won't even entertain the notion of doing it if it is going ruin anything between us. After a couple of days, I told her that I'd rather her not do it but I can deal with if she feels she has to although it may mess me up for a while. She really doesn't want to hurt me but I really want her to be satisfied and not have regrets or resent me later on for holding her back. I also feel very responsible for this because if I had just told her on new year's day that I wasn't okay with her making out with this guy, she would have been able to put him out of her mind much more easily than she can now, being she's already had sexual contact with him.
I fear that this subject has already changed our relationship somewhat but we are both terrified of losing each other over it. Do I have any right to demand that she not go through with it now? Do you think she'll resent me later on down the road if I do? Any advice is appreciated.