If they equate sex to exploitation, then they shouldn't have sex to begin with.
Most Helpful Girl
Well, if a girl has sex with you because she thinks you love and value each other and then you turn out to just be looking for sex that's a situation where a girl can feel used.
Or like if you have consensual sex with a girl and then treat her like your penis made her dirty she can feel used.
There are loooots of situations that happen between people that can make one or both parties feel used. There's not many blanket "everyone shouldn't do X" statements because so few apply to everyone.4
Most Helpful Guy
Because they feel guilty. Many girls aren't really taught to take responsibility for their actions, so if they regret having sex with a guy, they feel better accusing them than actually taking responsibility for screwing up.1