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.
Yeah, I figured you would have something to say.
People have a right to do whatever they want with their own bodies. Including leave relationships they aren't happy in. To some people physical attraction is a big part of that. (Not making any moral statements about whether that's good or bad)
There may be any number of other reasons people are unhappy in relationships including a partner's or their own changing habits and lifestyle.
But I think that intentionally doing things you know will make your partner see you as unattractive is in some way disloyal to the relationship. Everyone has a different tolerance for what they're willing to accept. For some it may take something extreme like substance abuse, and for others maybe it's a number on a scale - shallow or not.
Also I think that sometimes when a person decides to let themselves go as we say, what they really mean is they are letting go of the pressure and responsibility to please their partner because they're giving up on the relationship in some way.
If OP really wants advice whether or not to gain, I'm sure the opinions coming from this particular site will be heavily skewed towards gaining! Seems to me more like looking for support on a position already chosen. I think OP is likely already dissatisfied with said relationship and that's why strongly considering "letting go".
Would you be with someone who couldn't love you if you got fatter??