What you are talking about is something many gainers have to deal with.
For myself personally, I know the attraction comes from the actual process of weight gain, going from small to big and bigger, rather then any final number or size, so if I was ever to pursue it purely for a sense of satisfaction, I think I'd end up gaining forever.
If you think this is the case with you, then you need to be more realistic about it. Consider “maximum weights” and such, not as goals that you’d be happy reaching, but as points that you’d rather not go past due to how it would affect your life.
Given the opinion of your fiancé, that shouldn’t be too hard to sort out, lol. Just be aware that when you do reach them, you may want to continue gaining. In which case, rather then impulsively following the desire, make sure you re-evaluate your previous gain, your current weight and how further weight would effect you.
I think this is a point where a lot of people blur the line between fantasy and reality; of course it sounds fun to just give into the idea of gaining and growing indefinitely, but it’s not that simple, especially when you start heading into super sized weights.
I wouldn’t worry too much though, you’re aware of it now before you have even really started, which should help you keep things in perspective. Ultimately, you should just enjoy it though, I’m sure between yourself and your fiancé you’ll be able to keep it under control.