Hmm....the human organism...
Depends what you want to call fat?
I really have no idea how to answer this as I'm.....phew......(bout to roll out the stinking adjectification on myself - never thought I would) pretty solidly in the 'demisexual' category so because of that it takes a lot to get me interested in someone to begin with; that also means that I don't really know how much more or less leniency their weight range would have on me before I tap out.
At pedestrian level - its a rare girl whose more than 20 pounds overweight and still fills it out well enough to look attractive to me. If I was ever with someone and had to break it off with her because she weight gained too much its not a 'fat people disgust me' thing, its a 'I'd love to keep this relationship going - I love who you are, but when I can't bring myself to f' you anymore and it boils down to once a month - done drunk or badly and the whole relationship starts tumbling straight down hill because you feel like I'm holding out, don't love you, am not interested, etc. etc. - I'm at the top of the hill looking down at that smouldering car wreck with us in it and would rather call a halt before we hate each other'.
Clearly if I'm really into a girl and I feel like she's within 5 or 10 lbs of turning my brainstem off and making sexual contact like pushing two strong positive magnets together - I'm going to do what I can to cook for her, swing her habits, work out with her, not overdo it but really try to say in so many nonverbal ways 'I hate to say it but you're hedging into dangerous territory - I want to bring you back from it, be positive peer pressure, and help you'. She'd have to be willing to go with me on that, if not....we'd be looking at the scenario above - ie. slow downhill progress, her hating me for not banging her, etc. etc.. I don't want to be in that kind of relationship, I don't want to wish it on another person, and I'd clearly rather be with the type of person where if she or I really start going astray in some direction where we're endanger of breaking it that we're both willing to let the other know what's not working or how it needs to be curbed.
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The loneliest part of life: it's not just that no one is on your cloud, few can even see your cloud.