I know a newlywed couple who have sex less than once a month because of this -- he doesn't respect her, she knows it, and she doesn't trust him, so she doesn't want to give herself to him.
If you have a girlfriend now and feel that way, think of how much stronger that feeling will be with your wife someday. That's what I mean by "sex killed my best relationships." People can relate on many different levels -- emotionally, mentally, physically, spiritually.
But when my girlfriend and I started relating mostly physically, it short-circuited the other parts of our relationship.
It's like no one wants to acknowledge that it's happening, even though it is. Because we'll go into the marriage with me having more respect for her and her having more trust in me.
One thing I've learned: if a girl doesn't trust a guy, she doesn't want to give herself wholly to him.
The two things were this: 1) I lost respect for the girl (even though I didn't want to); and 2) she began to mistrust me (even though she didn't want to).