Caveating this with I don't actually know who this person is, I just followed the link in the plus size thread
But on your point about fatphobia - I wonder if that comes from people feeling like it's fetishising? I.e. the only average weight or muscly men who go out with fat women must have a specific thing for fat women, rather than just liking the person they're with for the person they are, regardless of size.
As a sample of one, as a fat woman whose in a relationship with a slim man, it's the sort if thing that's bothered me in the past. Probably a reflection of my own insecurities but I've been worried that people might think my partner is with me just because he's got a thing for fat women (which he doesn't), and I've worried before that people might talk about it/laugh about it behind his back.
Obviously some people do have strong preferences and there's nothing wrong with that. Generally, 'fatphobia' as a reason to be outraged often makes me cringe and roll my eyes so they probably are being unreasonable in the comments