Cheating liars are everywhere. OP did nothing wrong. Hookup culture and liars are a bad mix and they're destroying lives every day.

Speaking as the GF in this exact scenario. Found out after three years; I was being gaslighted by a closets bisexual man who is also a sex and porn addict. Hookup sites are inundated with these folks.

It's traumatic for the partners when they find out. It's unfortunate there's no way for hookups or the family to know about it prior, until they have done profound damage to other people across the board.

It's messed up and it's also becoming more and more prevalent.