Don't tell me I'm not fat!
This comes from guys who don't necessarily have a problem with my weight -- they just have a problem with fat people.
When you say, "But you're not fat!" Or worse, "Oh you're so (beautiful, smart, sexy, whatever) for a fat girl…" all I hear is how much you hate fatness.
I am fat. There is no denying that fact. When you tell me I'm not, what you are really saying is that despite my body size, I'm not all those horrible things you tend to associate with fatness. "You're not the kind of lazy, stupid, disgusting, [insert sizeist insult here] slob who I expect fat people to be," is not a compliment.
Don't make assumptions about my ex- (or current!) partners
Dudes tend to assume that I haven't dated a lot in my life -- or the opposite, that I'm always up for casual sex because I'm desperate for attention. They often come into a relationship believing that my past partners have been abusive or unfaithful, or that my current partner isn't interested in sex and that's why we're poly. Basically, that he's arrived to save me from my terrible, sad, fat life.