It sounds like it might possibly be a case of J being with you sexually because you and G, your primary, presented yourselves as a package deal. Is it that you only wanted a girlfriend to be with you both and required that she be with both of you, rather than see if she wanted to date one or the other of you separately? If that is the case, and she was really hot for G, then she might have felt that fucking you too was the only way she could be with G - the one that she is really into. She might not even be bisexual, or would just prefer to be friends with you but saw that in order to have a more romantic and physical relationship with G, she had to "do you" too. Or it could simply be a case of attraction and connection between you and her having its own ebb and flow - it can't always stay the same all the time.
I am just guessing, of course I don't know - but it sounds like a conversation might need to happen among all three of you.
I hate to see people sell themselves short like that and go along with having sex with partners they would rather not have sex with, but we've seen this sort of dynamic happen time and time again here. It usually seems best if a couple dates separately because this seems to be very common. It's just nigh on impossible for two people in a couple to find one other person who will relate to, develop feelings for, and be sexually attracted to them both all equally.
Hot chick in the city.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein