For me, I found that I felt guilty any time I would do anything sexual with someone. I kept feeling this way until I met my girlfriend and for the first time I never felt guilty about sex. so in my case, I feel guilty because I didn't have powerful feelings for the person. With my girlfriend, I have strong feelings of love which makes me feel like sex with her is "right". This may not be the case for you, but personally I can't commit sexually unless I have an strong emotional backing.