On the other hand, callous sex probably causes a significant fraction of the misery in this world. Note that this does not mean you have to believe you're in love every time you have sex. If there are no tender feelings on either side, the sex is likely to be mechanical and poor. If she has feelings about you that you don't about her, having sex with her is likely to leave her feeling unfulfilled and used, and can be cruel enough to leave emotional scars. You wouldn't want this to happen to you, so don't do it to her.
Women can sexually use vulnerable men so unfeelingly that the memory is a permanent ache.
It's less common than the reverse because of the different ways men and women are socialized, but probably hurts men all the more because it evokes emotions that a man has much greater difficulty accepting or acknowledging. One kind of callous sex disregards your partner's rights and feelings. Another kind is sex that disregards a third party's rights and feelings. Accordingly, you should respect the sexual and emotional boundaries that people agree to in order to protect the relationships that they value. Nor is it good to help other people cheat on their contracts.