Sometimes I just check out a girl. I try to do it quickly and discretely. What annoys me is when my dad looks at her as well. This feels like an invasion of privacy. My mom has done this as well. I've checked out girls a few times and my mom would comment that she's attractive.
Sounds like they are accepting and supportive of your orientation. Maybe they do that to show that they have an interest in your...interests
If your mum says stuff like "She's attractive." I think that it's just her way of trying to show you that she is supportive of you. It's quite possible that she may be feeling as awkward as you are feeling when she says it. I can understand the feeling you get when your dad looks at the girl you are checking out. That would annoy me as well, but I guess he could just want to get an understanding of what kind of girls you are interested. Sounds weird, I know but the important thing is that they don't seem to judge you for it.
My mom does that with guys My boyfriend does it with girls but it's usually because I'm obvious about it I think ur moms just being supportive though