Why do guys only want to be friends with benefits or have me as their side girl?

I'm going to go out on a limb here and say that it could be because you hold yourself in such an elevated state that men tend to feel too insecure to engage in serious relationship type contact. It could also be that this attitude, together with the feminism thing, just makes you a bit less interesting as a life partner.

For life partners, you'd want someone that makes you feel awesome, not someone that think that only they are awesome and everyone else can take a long walk on a short pier. (Not saying that's how you are, just speculating). But if you're hot, I'd still want to bang you even if I don't want to live with you. Hence side positions.

You could also be the sweetest and kindest person around, and you simply haven't met your match. It's no wonder that any man would like to have a sexual relationship with you then, but simply feel that the personality type doesn't match for a full-fledged relationship.

They just want to bang you i guess

I'm happy to be a friend without benefits. Some of my best friends are without benefits of any kind and we get along fine.
One of my pet hates is asking a girl for favours. It gives me the creeps big time.
I'm a bit shy you see.
I've found that if I respect a girl, take her on different and interesting dates like horse riding, gliding (I'm a pilot) sailing etc and we can laugh along together, treat her like my best friend, share some fun, then drop her safely to her door and bid her goodnight without intent or prejudice and be off into the night without delay, eventually she will take further interest....or not.
Usually though there comes a time when I'm dragged backwards through her door and tied to a piece of furniture so I don't escape while she slips into something more comfortable.....but It wasn't my idea, and now we're best friends, with benefits.