how does Stan feel, knowing he tore Ford away from a husband and child? How does Stan feel that his brother got MARRIED?
I'm still trying to figure out when and how Ford exactly tells Stan about everything, but I do know that Stan would feel immensely guilty about it. However, he's not outwardly going to show that. Instead, he'd deflect and say that there was no way he could've known that. Basically, he'd act very dismissive and deflective about it just to hide the guilt he would feel over it. Stan would also feel curious about the family that Ford had made for himself, as well as slightly sad that there was never a chance for him to meet them, but those feelings would be pushed even further down than the guilt.
The last thing Stan had ever expected was for Ford to get married, to anyone. Especially not when he was in the portal. He'd be very shocked over the marriage, but once again, Stan deflects. He'd make a ton of jokes over the fact that Ford was married to a fish alien. Trust that anytime fish is brought up in any capacity, Stan will be making a joke at Ford's expense.
Moral of the story, Stan can (and will) deflect any serious feelings that he has around Ford's alien fish family.