It's not about gaining a few pounds. It's about not caring about staying attractive for your partner. If you take care of yourself when you're single but then when you're in a relationship you let yourself go, I think it's disrespectful. I've literally heard people say before " well now I'm married, I don't need to bother anymore so just eat whatever I want".
It's not even about the physical side of it, what I would find unattractive is the lack of initiative to stay attractive. Just like if my partner said e.g. "now I'm in a relationship,I don't need to try and be nice anymore".
Not saying that everyone should be bothered about their partner gaining a few pounds. As I said,life happens but obviously if that person asked the question,then it must bother them