You’re willing to settle/share
You know what I think of when I see those Instagram posts of the girl trying to sneak a photo of the guy, but he has his head strategically covered, and they have the caption: “What’s understood doesn’t need to be explained”?
“You poor, unfortunate soul!”
You shouldn’t have to settle to be a side piece. Heck, even the “main chick” position is settling. Instead of being happy that you have a mate, why don’t you just find one who makes you his/her one and only.
Unless you like that arrangement, and if so, carry on!