Your needs are incompatible. It's that simple. It's very sad when that happens, but he's polyam and you aren't. You can't make him be monogamous. He can't make you be polyamorous. You don't want to date other people. So don't. You seem unable to adjust to his seeing other people (which makes sense...he's cheating). You don't have to do that. No one has to be the bad guy. But if you want to assign blame, he's the one who knowingly hurts you over and over. I hear you admitting that you aren't perfect. OK. That's fair. Neither of you are good to each other. So split up. Life is shorter than you think, and the sooner you do it, the sooner both of you can find partners that ARE compatible. It's sad that there are kids involved, but if you are both unhappy, they will be too. I do NOT mean to suggest it will be easy and rainbows. It's hard to divorce/split. But it sure sounds necessary in this case.
In rereading, I just wanted to add that divorcing doesn't mean abandoning your kids, at all. My ex-h and I split in 2008, divorced in 2011 and we have both remained supportive committed parents to our three children. We even continued to both attend birthday parties and did Christmas together the first couple of years. Our kids were older when we split, so we didn't continue to always do birthdays and Xmas together, but we would have if they were school-age kids. But separately, we cared for, and still care for our kids.
I am confused who is LGBTQ in this situation? OP, are you saying that your partner wants an open relationship in order for him to explore his bisexuality, to date other men?
Or that he is pressuring YOU to have threesomes with him and his female partners, when you are not bi at all?