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There's never a reason for details of a partner's past sex life. It's one thing to ask number of partners, condoms in the past. No other details should be asked for or volunteered unless you've been married a long time. Asking for details is a sign of someone looking for something to nitpick about.
When he started nitpicking you, LW, the relationship was over. People nitpick when they want to get out but don't have the spine of balls to break up. They want to drive the other to break up so they can play victim and say they were dumped. He's mentally checked out of this relationship. The nitpicking and using the three way to manipulate you are proof.
For me, a single person having a three way is neither a slut nor proof that a person does not value oneself. An adulterous affair, staying with an abusive person, drug use, etc is proof of not valuing oneself.
It used to be premarital sex made a woman a slut. Then it was oral sex. Then it was anal sex. Then it was lesbian sex. Then it was allowing yourself to be photographed. Now it seems to be a three way for some people.
Give it another ten years. It will be virtual sex or posting videos of yourself having sex online...oh, wait, people already do that. Ten years from now, LW, you'll tell this story to your friends and they will all laugh at him.