Sexy is a quite broad word that probably used by different people in different ways. I think for most people it about what they feel when looking at the person. Those feeling where set up by evolution over large time frames.

Evolution doesn't really care about whether you get a fun intercourse partner.

But it's not only evolution. It also has a lot to do with culture. Culture also doesn't care about whether you get a fun intercourse partner. People who watch a lot of TV get taught that certain characteristics are sexy.

For myself I would guess that most of my cultural imprint regarding what I find sexy comes from dancing interactions. If a woman moves in a way that suggests that she doesn't dance well, that will reduce her sex appeal to me more than it probably does with the average male.