It really doesnt matter what it is you want; it's always ok to ask.
It's ALSO always ok to decide NOT to have relationships with people who don't want what you want.
It seems obvious with kids.
For example; If someone WANTS kids and they meet someone else they are attracted to and start talking. They tell them, someday they want to have kids. That person says "not me! OMG I NEVER want kids." It's TOTALLY ok for EITHER of them to decide "not a good plan to date each other." In fact, it makes PERFECT sense.
It's always ok to have your own preferences, state them clearly and ask the other person if that is something that works for them.
It's NEVER ok to try to force someone else to live by your preferences if it's not what they want.
SO if they don't want kids but you do-let them go. Don't try to push them to change their mind.
Likewise; if you meet someone who wants an open poly dynamic and you know you want polyfi-don't push them to change for you. Just don't DATE THEM.
"Love As Thou Wilt"