This is exactly like my situation. Until now, I stay 1 or 2 nights a week in bf's house plus a few hours during the week. I feel I'm more bonded with bf sexually than partner. Couplers is right. There is no perfect person in this world. We love the qualities that are different in both men. We love them despite....!
It can still work out in spite of your families being wired to monogamy. There is a need for hard work though. Like, always planning for discreetness, and managing your time effectively between the 2 worlds.
But unlike Couplers, I'm still in the beginning stages of the journey and am not sure if I can keep everything inside the closet. I'm lucky that partner and bf lives in separate but nearby cities. Bf' family and friends know me there, as well as partner's. It is like living in 2 realities. Eventually, if bf will be raising a child with me, things might change, like, living arrangements, "outness", etc. You should consider these things too.