Adoption agencies are getting far more liberal. I’m pretty sure that those which receive any state funding are no longer allowed to discriminate on anything in the various Acts that disallow discrimination, so sexuality and gender terminology are protected. Nonmonogamy could be argued as an “unstable environment”, sadly, even in the presence of overwhelming contrary evidence in any individual case, so that could well count against you for now, assuming you were honest and open about it.

That said, I’m aware of a nonmonogamous gay couple who successfully adopted despite the agency being in full knowledge of this, so it’s not impossible. If the time comes: good luck!