Why are one night stands easier than sex in a relationship?

I can have sex with on a first date no problem. Maybe even friends with benefits, no attachment whatsoever. However I started dating this guy recently, we had sex a couple times, I enjoyed it. He asked me to be his girlfriend and I said yes of course, he’s great and I really like him. We had sex that same day he asked me to be his girl and it was great, until i orgasmed. I felt absolutely disgusted in myself and I have no idea why. It gave me flashbacks to my last relationship, it was the same problem, sex was fine until we had built a deeper connection. It seems like the closer I get to a guy, the more gross I feel having sex with them. Why could this be and how do I overcome it?