Did you have any life experiences that made you start thinking about this kink?

For me I think one major one was when I broke up with my first serious gf, she went and had sex with my best friend to try to make me jealous.

It definitely worked to an extent lol, even though she was my ex at that point I was fuming about it for a minute..but ultimately I got over it and stayed friends with him.

More than anything I actually found it hot as fuck imagining him fucking her. This is probably why ultimately once I finally got in a relationship with a girl I felt comfortable exploring this kind of kink with and sharing, he was the one I ended up sharing her with. So in total he's fucked 2 of my gfs, both of which had sex with him due to my actions