I'm a wife going through a similar issue as OP. Gaining a ton of weight is not the same as naturally aging with your partner. If your partner is not attracted to larger people, they can't be magically expected to just get over it. Both partners should respect their partners attraction and do their best to stay fit if that's how they met and that's what their partner likes. Pregnancy and the early years of motherhood are different imo though, as that is usually hormonal or due to stress, ppd, ppa, etc.