They're stereotypes. Not true 100% of the time but there's a reason they exist.

you can have a 4.0 and party on the weekends

If you're the studious type you've either sacrificed some part of your social life for increased academic skill or actually have stuff to do on the weekends, like homework, depending on your classes.

you can have "meaningless sex" and still be class president

I mean honestly I don't understand why "meaningless sex" is a bad thing. If we should have only meaningful sex we'd all be waiting until marriage. Unless you mean that he's slept around, which I mean to each their own, but given that StuCo/class presidency is just a giant popularity contest, it really doesn't matter.

a girl can wear makeup and flirt with guys and still like programming

Yeah? Of course they can. People are wary of those kinds of people, though, because they tend to be interested less in stuff like programming and more into like... arts and social stuff. It's also a stereotype that people who spend all their time on the computer (programming or whatever) are nerdy and introverted, so those naturally clash.

Stereotyping people is just an easy way to categorize people without getting to know them. It's harmful sometimes, beneficial other times. I don't think anyone would care if you're "outside the box", they might just think "hmm that's cool". Except for your second example, some people would understandably not like a guy who sleeps around to represent their grade.