Most Helpful Guy
Many mammals we're related to, females carry fat on the butt (where men don't) as a way to visually advertise that they have sufficient fat stores for fertility. Males find that attractive. In nature, being obese is rare, a bigger worry is being too skinny to successfully have and raise a child.
There is a theory that human females started developing fat filled breasts (breasts are not large because of the milk glands unless actually breastfeeding, most other mammals the breast area is flat) when they started walking upright, as a way to provide the similar visual curves the butt gives from a front facing angle.
Anyway, they are signs of fertility. Guys aren't -thinking- oh she looks fertile , but they instinctively find it hot, because guys who slept with girls who looked like that in the past had more kids, and so their preferences spread.2
Most Helpful Girl
It's not just skin to them.
Those body parts turn them on just because.