Wow! My first reaction is stop having threesomes with them right away! Except on those once a month occasions when you're TRULY in the mood. Simply be honest about your reasons, that for you sex is something you prefer to be with just one other person, except when you're feeling unusually uninhibited. Maybe leave out the part of her being competitive in bed... the other reason is reason enough.
As for the other thing (no sleep overs), my heart goes out to you. That sounds very painful, and must take a lot of strength to cope with.
Personally, I could not handle being second class in a relationship with someone I deeply loved, nor would I feel ok with making my partner's other partner second class... even if I was suffering the pain of jealousy I would not want to make someone else suffer the pain of being second class just to spare me from the jealousy. That's immoral in my opinion. That doesn't mean I don't think that there can be a healthy and respectful poly relationship with primary and secondary status. But we should all prioritize how to make sure that each of us suffers as little as possible
On the other hand, each poly relationship will have different boundaries or rules and I try to understand and respect them (within certain limits).
I can think of only four options for how to deal with this:
1) Try to persuade her directly. (I know you said she's stubborn but if you tell her how much it's hurting you, she might surprise you with compassion. I know that when I've been jealous of my partner's partner, thinking of how she too can experience jealousy and insecurity and loneliness and etc. helps melt negative feelings towards her into caring and sympathy.)
2) Try to persuade your boyfriend to try to persuade his girlfriend to change that rule, or be more flexible about it. (Maybe he's already tried doing this, but if you tell him how much it's hurting you, he will hopefully keep trying and trying, and maybe even insist)
3) Continue with things as they are
4) Break up with him
I wish there were other options but I don't see any. I hope one of the first two options work! I would try persuade her directly before trying to get him to do so.
Good luck and keep us updated
Last edited by ultraviolet; 08-19-2012 at 08:18 PM.