The OP said she isn't a sub, she is a girlfriend. You saw that right? She doesnt have a PE relationship with this guy. No idea what they do in the bedroom butshe said that she is his girlfriend. OP is his girlfriend, other woman is his sub. OP thought that all subs would automatically be beneath her place as girlfriend, she found out she was wrong.
Kinky sex is all good but many people want the wider relationship too, so regardless of what OP does in the bedroom, someone who is up for a PE will have added compatibility. So no, someone who doesn't have PE relationships but does have kinky sex isn't necessarily vanilla but they also lack the added compatibility someone who wants PE relationships seeks.
I'm not saying that the OP has no place in his life anymore, but it isn't surprising that he, a Dom has found a compatible sub, and has fallen head over heels.