system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the psychology behind why men get more aroused by the thought of catching a glimpse up a girl's skirt, but seeing them at the beach in their bikinis is no big deal?
gpt
There is always thrill in seeing something that you are not supposed to see.  That's why girls in PJs are often so sexy to guys.  It's a peek at the inside world, the world that is foreign to us and that we are not apart of.  A peek at the private life if you will. 

Bikinis are public, not private.  A skirt that's riding high is a peek at something that is not public, it's a peek into the private and that titillates us. 

The mental component of attraction is important for men, not only women.  It just functions differently between the genders.  In this case, the thought that we are getting something that's special, or just for us, or explicitly not for us,  is what gets us going.