Delta9:
From another point of view:
A person's body can change a lot faster than their partner's preferences can keep up. Small changes are bound to happen slowly over time. That's just the process of aging and everyone knows that. If you're really in a long term, committed relationship then I think it's the responsibility of both partners to accept and account for this reality. No one stays young forever.
However, when you make the conscious decision to drastically change your body in a way you know won't be attractive to your partner, a person who you supposedly care about, I think it in some way violates the implied terms of the relationship. Nobody wants to sign up for one thing just to get the bait and switch later.
If I love my partner, don't I have some responsibility to at least try and maintain my body in a way that will be attractive to her? And of course there are any number of ways a person can change themselves, not just getting fatter or thinner. Drugs and alcohol are another way of destroying your body.
If you choose to put yourself and your own desires before your relationship and your partner, that's fine. It's not a wrong decision. But maybe you should talk to him first or simply end the relationship instead of leaving him stuck with someone he doesn't think is attractive anymore, forcing him to make the choice to leave or not.
Munchies:
Not you likening weight gain to substance abuse.
That's not cute, sweetie.
Look, if someone gains or loses a lot of weight by choice, but those gain's aren't impacting your life significantly, then that person is shallow.
If you are in a relationship that's based on being active or you are neglecting your relationship in favor of getting fat, that's one thing. But if the only thing that's changing about your relationship in a number on the scale, then the person who wants to leave is a shallow piece of crap.
Many relationships are very shallow, but also comfortable enough and fun, or at least something to do, or at least better than the idea of being alone. We don’t know anything about the OPs relationship, other than that they seem willing to get out of it in order to gain more weight.