Look, mate, if you find a partner who is already into what you’re into when you meet them, more power to you. If they wanted to gain weight BEFORE you put the thought in their mind, then more power to both of you.
I, personally, find the idea of wanting/asking a real person (not a fictional, idealized person) to physically change themselves and/or adopt and reflect your desires… Well, morally questionable, at best.
If they agree to a suggestion, under no duress and in the belief that you’ll love them, regardless of their choice, and that they understand the full ramifications of what they’re agreeing to in the first place… Then I certainly can’t hold that against anyone.
Regarding your “assumption” quibbling: It IS more likely than not one would end up regretting this decision if taken too far. Joints ache with more weight, blood doesn’t circulate as well, breathing becomes harder, movement becomes slower, clothes become tighter, sweating comes easier, bathing becomes harder… There’s a lot of downsides, believe it or not. There’ll be a lot of times they won’t really like what’s happened to them. This really isn’t something you just casually ask somebody to do. This isn’t the same thing as slipping on a costume, or using special lube, or something. This is a genuine lifestyle change.