Who's 'they? Assuming girls, you can like it or not, but good luck changing things. The sexual interest is most certainly not moment to moment - they decide you're interesting in a sexual dimension or not. Once you're in the 'not' bucket (aka friend zone), good luck getting out of it. She may not decide to pursue anything, but you have to be a potential first if you want a decent chance at sex. Whether you decide to pursue longer term stuff is a separate matter.
And yeah, girls go for confident people in general, which means assholes have an advantage, at least short term. Me, I'm finding that having fun and not worrying about specific girls attracts more girls than focusing on someone specific.
Now, some specific advice: find what makes you drool (aside from girls), be it, own it, make it work. Go to the gym, get in shape, wear clothes that look good on you and make you feel comfortable. When you see someone that interests you, approach them, talk to them, and find out what they have to offer. If that makes you nervous, talk to more girls with no particular goal in mind and get used to doing that. If she isn't interested, move on.
A good footnote to this: get good at gauging interest and nonverbal cues so you don't waste time on someone that just doesn't care. There are lots of girls out there, so don't worry so much.