Help...

MJ
I don't really enjoy sex... 
I've had multiple sex partners so it isn't who I am having sex with and I'm currently in a serious relationship (we even live together) but I honestly don't get any pleasure from having sex. What is wrong with me? I feel bad... I will do it, of course to make my boyfriend happy, although he never wants me to do it if I don't want to but I honestly never want to. I just don't find pleasure in having sex.