I look at a girl from top to bottom and then make a call based off of that.
Nice hair, cute face, nice looking boobs (they don't have to be big there just needs to be something there), then I go down to her waist (I don't mind a girl with some curves but I have a limit, and I definitely don't like a girl that's skinny as hell), then move to the legs (I like average thighs, nothing to big but nothing skinny) and then ass (if there's a curve then great but if its flat then it would be minus points in my books)
Don't get me wrong I appreciate a woman's body but just like girls and guys I have a preference and something on her body would be a deal breaker even if she had a personality that I enjoy. I need to be attracted to both their personality and their body in order for me to really be fully attracted to her.
The butt isn't the deciding factor for me as I consider everything from looks to personality. But looks wise, she has to have something on her bottom and not just skin and bone. Breasts can be average, and a curved body would be nice. But again, that's only physical attraction(i.e. LUST). She has to have a good personality along with the nice body for me to truly like her to the point of asking her out!
If its too flat then when you slap it against your hips in sex it will hurt. If its all muscle and big then it will make you come when you pond it. Boobs mater too. but if you have personality are kind and make him feel like a man, then any fit young woman can have any man she wants