Wow -- what an interesting question! I've often contemplated starting a thread on this but kept putting it off.
Overall, my wife is supportive. Although she's not interested in MRR at all, she does show an amused interest, especially in how I expand and tweak my layout.
Admittedly, I have a tendency to over-do things. When I'm interested in something, I go at it "full-throttle". So I really have to watch how much time & money I spend and try hard to balance things. Obviously, it's not good if I'm down in the basement 3-4 hours, several nights in a row, which I can easily do if I'm not careful! Problem-solving, especially, eats up a lot of time. I'm also not a handyman, so problem solving takes longer! Sometimes, I just have to decide that I'm not going to "do" MRR for awhile.
Having two young boys also prevents me from getting to my layout until about 9 p.m. I realize this will change in the future, and my boys may even be able to help when they're older -- they already show an interest but are really too young now for MRR. I actually got back into this hobby because we started taking our kids to MRR and real-life train events!
Like others, I also have to make time for the honey do list. On the positive side, it's good how I will take one or both of our boys to MRR events or for rides on real, heritage trains.