Big question!!! Anyway, I will try to give my small opinion about sex in couple. I think that the most important is that sex is like a game. If no game, no sex. But also, it's depend on the stress. If the guy stress in his life, this can stop his libido. His girlfriend, even beautyfull, cannot help him. She has to be aware that sex is not on command.
So never ask about make sex !! NEVER.
If she is beautyful, she can make him jalous, this can make his libido up, or make him a very nice massage without sex, only for relax. Then He will feel better and maybe ask about sex.
But, before I can say more options, I have to know how old he is, does he is studient or work... Does he stress in his life, maybe he has complexes, many things can explane his attitude.
Tell me more, I will answer.
Nobody is perfect - I am Nobody