Did he have to use the L word for sex?
Guys know that saying I Love You can give them the best chance at getting sex.
It isn’t a guaranteed magic line, but it can push her over the top if you actually have any chance with her.
Did she have to use sex to get what she wanted from him?
She might want your money, or what it can buy. She might want the social status if you are a jock, or have social standing in a group that she wants to be part of — the in group at school, a member of the yacht club. Maybe she just wants someone to say that she belongs, that she is smart and pretty.
Whatever the trade off, you are giving away part of yourself in trade for something. Guys — you become a liar to get sex. So you get the sex. You also become a liar. If you can lie in one area or life, it becomes easier to lie in other areas. Girls — you aren’t being truthful either. When you trade sex for whatever, you are going to being telling him how good he is and how good it is for you.
Whenever you trade the truth for a lie, bad results follow.