Another thing, being 25 and not having (I presume) had a relationship with a man with kids .....
You're not realising how difficult and unpleasant and stressful that can be.
I've done it. Different circumstances (not like his behaviour) but - it was not easy or nice at all.
And they weren't even that bad. Only one was really antagonistic.
If they split and you got involved with him and stayed involved, sooner or later you'd have to spend time with them and eventually live with them.
Most kids in that position just don't really want you in their space/living with them and it will come out in.all sorts of ways. It's not their choice, it's forced on them These kids will also be suffering from their family breaking apart.
If you are younger than their Dad, it's even worse.
I was "only" 9 years younger and I felt it.
You're 20 fkg years younger.
If you think it would be pleasant or easy, think again.
You would be much better finding a guy without kids, rather than having to deal with that.
Then there's money ..... He'll be trying to scrape money for you and any kids you had, after he pays for his kids. You don't have to be in that position, you wouldn't be with a bloke without kids. Where all (both) your money & effing goes into only your kids together.
Having experienced a relationship like that, I would choose to have kids with a guy without previous kids 100 times over than one with kids from a previous relationship.
Another point, you haven't had kids yet so you don't know how challenging it is to go through birth, recover from it and then deal with the early, demanding, stressful, sleep deprived baby & toddler years. You feel vulnerable and stressed. You need a huge amount of privacy, support, tolerance etc. and you wouldn't get it with step kids/teenagers around.
Having left that relationship and had a child with another guy (who hadn't got any kids from a previous relationship).it just wouldn't have worked. It would have made an already "challenging" time much much harder, with ridiculing, giddy, eye rolling, immature teenagers around.
I would choose a guy who's not got kids from a previous relationship a hundred times over than one who did.
And that would be if both guys were good guys. Not the case here. You think he's a good guy because of his excuses for his behaviour.... But he's not.
Decent 39 yr olds do not get involved with 19 yr old girls, let alone when they're married.
As I said, he's slam far more likely to produce kids with ASD at his age.
Find someone closer to your own age. Don't get dragged back in with this messy, odd ball, predatory, cluster fuck guy.
He's hanging around you and trying to emotionally manipulate and pull you back on cause it looks like he might end up on his own. He was happy to let things go and stay married (with only occasional contact) when he wasn't facing being divorced and single.
He's no soul mate, he's predatory and opportunistic.