I think this LW needs permission to dump him, frankly. I can see EGS' point that it's a somewhat positive sign that he still cares about his previous families. And normally I think it's unreasonable to expect full disclosure about your entire life history in the first weeks of dating. If his ex-wife was someone he foolishly married in his youth and they realized their mistake after a few months, well, it's good to know but I wouldn't call it 'concealment' if it wasn't mentioned immediately. But he's the father of three teenagers, and a committed father by this account, not just a provider of child support payments. That should have been brought up as soon as they started talking about the possibility of a future together.

I am very sympathetic to this LW who has been single for 15 years and desperately wants to find a life partner. But people in that situation are targets for exploitative men; I don't know whether the guy is actually currently cheating, but he is love-bombing and it sounds like he's at very best less invested in this relationship than LW. I think she's better off without him, and making plans to be a single parent, not least because that's probably going to be where she ends up if she does have kids with this guy.