I shall reply as an avid butthole enjoyer.
I also really love anal, but OP asked about why men loves *buttholes*, not anal.
Let me tell you:
It's hot as fuuuuuuuck.
Yeah, sure, pussies are great. But they're "normal". They're "expected". Of course a man will like and desire a pussy.
But the butthole? That's some connoisseur stuff right there.
It's cute, it's inviting, it's delicate. The way it winks and clenches.
Also the biggest thing is the fact that girls are ashamed of their butthole. They don't want their buttholes to be looked at, or appreciated, or god forbid touched or played with or licked or fucked.
It's forbidden, it's embarrassing. A girl doesn't want people to acknowledge she poops. It's unbecoming. So women protect and guard their buttholes, and are shy and embarrassed about them. I mean, it's true, they poop from there.
But that's what makes it so hot.
This is why buttholes and anal are so hot.
The main reason.
Because girls are embarassed and shy about them.