Most Helpful Guy
First thought: "those leggings are hot." I'm a leg guy, so I'm turned on by the shape of a woman's legs. I will absolutely stare at her legs for a bit.
Second thought: "I wonder how much of her vaginal area I can make out." I'll try to position myself to look at her crotch, if it's possible to do so without being obvious. If it's not possible, I'll move on, because I'd rather miss a great view than be caught.
Best case scenario is that she has a nice cameltoe, at which point I'll get as many looks as possible. Worst case scenario is that the leggings will have a crotch lower than her actual crotch, in which case it quickly becomes obvious that nothing will be seen, and I don't bother risking a peek anymore. Middle-case is that the leggings are fairly tight against her body, but no cameltoe is visible. I'll probably keep peeking to look for something, anything, maybe a mons veneris, or a crotch line; but I probably won't see much.
Why would I do this? Because male sexuality is centered around female nudity, and more specifically about imagining female sexual organs naked. If I can see something, like a cameltoe, then it will allow my imagination's naked picture of her in my head to be better informed, and more accurate. If I don't get it, I'll still imagine her sexual parts naked, but it'll be a bit generic.
Most Helpful Girl
They aren't thinking much. Too busy trying to look at her arse