@justaguy I hope that it was that she DTMFA'd you. Because... ugh.
Never mind. She doesn't owe you motivation to be thin. And unless she really, really REALLY has a healthy sense of self esteem, however bad that 15lb made you feel, trust me it made her feel worse. Because she's the one that lives in a mysoginist society that thinks somehow her 15lb is anyone's business but her own.
You're not responsible for what is attractive to you. Your heartpenis feels what it feels. But you ARE responsible for the fact that your heartpenis's feelings happen to miraculously line up with societies unfair expectations for women and how their bodies should be, and are responsible for what you do with that fact.
Unless you are very lucky, chances are really good that women you date will not be able to fend off the slow gain of weight that comes with aging (and childbirth, illness, a demanding job, or whatever). And unless you stay an adonis for your lifetime, you will also have to accept women will either be applying a very different standard of worthy attraction to you, or expect some payoff for being attracted to a dude putting on weight over time. Such as a nice paycheck. Or a really stupendous personality.
I haven't met very many dudes who are so wonderful that they're wonderful enough to marinate myself in daily misogyny for the rest of my life. And I really like having my bathroom to myself.
PS: You don't actually get a cookie of sympathy for realizing this a fucked up way to feel. Maybe you should just sit with that bad feeling for a while and let it tell you something.