Gaining

Only if they got big enough that they didn't want to get any bigger, otherwise, no. :-)

I don't think that you have to worry about that too much.

Surely if you're doing what makes you feel sexy and your partner finds you sexy, you'd do more of it in the relationship if possible, not less.

Regardless, relationships often lead to weight gain even without the fetish factor. I can testify first-hand.

Usually, relationships where each partner appreciates and loves the other's curves can keep the weight around It has for me!