Because your imaginary playmate tells you so<quoted text>
I know this all gets confusing for you, so then you go and assume it must be nasty.
He is simply following the logical implications of your dumbing down marriage to 'two people in a committed relationship', an obviously prejudicial restriction for three people.
What does it say about your intelligence to swing from imposing an imposter relationship on marriage to denying others attempting something even less of a conflict with real marriage?
But hey, I'm no longer surprised by the depth of denial you live in.
We put up with your imposter marriage, no one is trying to take that away from you