Right, I risk outing myself here, but I got married (despite reservations) when we did not live together. It was important to xH to be married as I was pregnant, and it turned out he was something of a controlling person. For professional reasons, we lived in separate places, and my dc from previous marriage was in school here. He also had dc from a previous marriage.
There were other issues in the relationship, so I am not going to project too much, but essentially the relationship worked when we were living apart because we were living apart. He could concentrate on his career, come and go as he wished, and generally carry on much as before. For this reason, being a non-resident parent also seems to work better for him.
I am not going to discuss the financial risks, but for the dc, there are enormous emotional risks. You are talking quite cavalierly about the pre-nup being enforceable if you break up early in the marriage. What happens to the dc when they have been living with other dc as step-family and then their step-siblings are no longer there? It is not like a first marriage when dc stay together; 2 dc are going to go one way, and 2 dc another. The scenario gets worse if you add a baby to the mix.
Now you could say this issue is there even if you don't marry. But marriage suggests permanence to dc; and also, if there is conflict over assets, you are less likely to be able to sort child issues dispassionately. We were not even arguing over assets, but xH used the relationship with his dc/my dc as a way of maintaining control, to the extent that I felt the best thing was to quite simply keep my dc out of it. And throughout, that has been the very hardest bit. But the whole thing has been very damaging.
Obviously, this is personal, and I am debating posting it as you are clearly going ahead anyway. I would not do it again, but then the experience has put me off bringing anyone into dc life again. I would really be questioning why the question of marriage right now is so important.