It's child stuff. Look at the ages you have here. And yes 23 year olds, you're still a child.
When you've matured and dated around and slept around, you stop criticizing somebody because they're "fat" or "skinny" because you've learned that either body type can get you there.
It also stems from lack of sexual knowledge. Those skinny twinks will have a bigger penis 4/5 of the time compared to chubby, husky or muscular guys. That's why I go for twinks. Trust me, I've averaged 53.2 sex partners per year since I came out. I've also been abstinent for the last two years. Point is, I've seen a lot of men naked and the skinny ones rarely disappoint.
If you're worried about a lack of muscle or buttocks, get them to work out with you. You'll have nice penis and the buttocks to work with.
The final point is that body type doesn't matter because it can change. Skinny guy can get really fat or very buff and a buff guy can stop working out and get fat or lose his tone and bulk and just look average. That's why you need to accept and love people for how they treat you.
i've actually noticed way more dislike of fat people than skinny people. either way i think they're just shallow and focus on looks too much (as if they're all soooo hot, right)