I'm curious. As a thin black girl, I've personally never heard that I should date white men or that I look like I don't date black men (I'm not saying that it doesn't happen). I just wanna know what makes us supposedly so unappealing to men of our own race that people tell others that.
We've perverted it, but we are attracted to signs of fertility... breast & hips. Us brothers added the ass. So, if by slim you mean lackin' breast, hips & ass, that's why. If no, and we're talkin' slim elsewhere, it ain't bein' slim that's the problem.
No different the other way around. Again, it's been perverted, but women are attracted to protectors & providers. The less you fit that, the further from universal attractiveness you get.
In either case, preferences vary widely to the point that you have to be severely lackin' to be unappealin'. The problem is more likely mental/emotional. Are those slimmer women out, lookin' confident & approachable, or are they feelin' a way about bein' slim, poutin' or overcompensatin' in some way (clothing, make up, etc)?