I think it's a primal, partly subconscious thing that helps choose a partner that will produce strong descendants.
Some girls might just enjoy looking at a guy's crotch though.
Let me answer your question with another question.
Why do men check out a woman's chest, butt, or crotch if they're wearing tight or skimpy clothing?
I don't look at a guys weiner, it looks at me.
I just feel as though I have to look.. not comparing size or anything! Just curiosity for some girls:)
I think what the ladies are saying is it is not specifically to check size, but rather a general curiosity.
If you saw a hot girl and she offered to lift her skirt and pull her underwear to the side, I imagine you'd be interested, right? But not to check any one specific thing. You want to know everything about it ;)
I'm interested to know what, if any questions the women would have when they check, even though its unlikely to be answered through clothing.
My wife told me when she was interested in a guy, perhaps after they'd kissed, but before they'd gone further, she'd be wondering how big it was, whether it was circumcised, ... I guess those are the main things. But girls might wonder how hairy he is, if it has a curve/what shape it is, whether it sticks more out or up when he's erect, etc. What do girls here wonder when they haven't gotten in a guys pants yet?
When a chick passes by with camel toe, your first instinct is to look, same with girls I guess