The older I get, the stupider Sam becomes

To me it's the opposite: DeNiro is not up to par with playing such a clueless idiot. And the writing certainly doesn't help him.
It totally needed a better explanation of this flaw from a psychological perspective, like he's a shy guy, or was treated badly before, or whatever puts him in such a naive position against the power of the pussy. What is he, 14?
Or at least give Ginger some redeeming quality that really puts her above the rest for him, like she does something for him that takes her to another level, anything.
We never see that in the movie, it's just baffling and annoying to see such a brilliant man inexplicably destroy himself and everything he's worked for to go after some skank.

I've seen that kind of thing a couple times where people get into a relationship and they just don't get out of it and they keep doubling-down on something that a moment's thought would extricate them from ASAP. Most of the time when I've seen it, the lady ain't no Sharon Stone, either.

You're right, it makes no logical sense, but this kind of thing doesn't always make sense. He saw her, got turned-on, and then he just got comfortable and wanted to make a relationship work. The idea of going out and dating was probably a lot of work for this guy so he stuck with where he was, doggedly, until he was caught in it.

He thrives on chaos a little bit, and he likes taming that chaos. That must have contributed. He probably thought he could handle it, but he was in over his head (we see that theme a few times).

Then, because his business is shady, his world is shady, so he might have a different "calibration" and think she's not the worst money-hungry crazy out there.

I think a lot of it was "I can handle the chaos; I'll straighten her out. If I love her enough, she'll change." We see that all the time.

per the movie, I don't know how it went down in real life.

He made her have the baby first before marrying her, because I guess he thought that would keep her tied to him.
That didn't work out how he planned.

Ahaha, yeah that guy would be perfect for a loser role like this one!
I could totally see him as a spineless fool selling his soul for Sharon Stone, but also be an ace gambling expert slavin it for the big wigs. That's exactly what I mean, DeNiro comes through as a tough, intelligent, sensible technician, and he never transmit that weakness needed for this character.

I would go with what Ace_Spade described in a post above, like Sam's weakness is to insist on something he's never gonna fix. But like I already replied, the movie is too quick and never develops such a brilliant concept.

Like I said he thought that he could manage her and the whole thing was an arrangement versus true love which has been happening forever. Sam figured if he put some money in her purse that she could be the little woman in terms of cooking and cleaning and hot outfits and so on. He figured that was the best he could do. Further, being married was no doubt a status Sam wanted to maintain to keep the locals pacified given his image to them was not desirable to start with. Having a parade of call girls going through his home was not going to help that. The Midwest gangsters might have insisted on a marriage to maintain a facade.

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