Why do guys care so much about women's appearance?

[Nikia]

This of course makes sense when they are unattatched, looking for a potential mate. I am talking about married or guys in a long term relationship.
I have some married or otherwise attached male friends who are always talking about the way women look. They usually seem to put the women into two categories: ugly or "hot"/someone who they'd like to hook up with if they weren't attached.
Since I am a woman, this makes me a bit uncomfortable. I wonder what they think about me and what they say when I'm not around. I'd be offended either way. I am also appalled by this attitude. I don't know what to say really.
Strangely, they become paranoid and upset if a said "hot" woman flirts with them at a moderate or higher level or says that they are attractive.
I don't understand. It has also made me more conscious of what I look like because I don't want to be their "ugly "friend. On the otherhand, I'd be offended the other way too. How do I approach this?