Warning, I'm going to be blunt and potentially offensive. Bear with me.
The reason IRL men who are "passive," "unassertive," or "the nice guy" don't get the girl is because you want them to work for you. You ask for something for nothing. You are high maintenance and the one's most likely to pursue you are the ones who want something. It's probably not actually love or sex.
Every woman (or at least every woman I know), including the "dominant" ones, wants to feel desirable, wanted, and/or beautiful. An "active" man who is pursuing a woman fulfills those desires even if he's an asshole. A passive man requires a woman to put his desire before her own and is therefore high maintenance before anything's even begun.
Additionally, society puts great pressure on women NOT to pursue men and is exceedingly cruel and judgmental when this rule is not obeyed. For example, imagine being a woman at a bar who just wants to find a dance partner. Every time you ask someone, you get looked at and spoken to like a slut.
If you are a passive, non-assertive, nice guy and you're into a girl, get active. Show her. Don't stop being yourself (that's just lying to the both of you and will backfire horribly unless your goal is just to have a one night stand, but even then it'll still probably suck), but one moment of courage to open up and SAY you're into her, sexually, romantically, or both, is far more attractive than you might think.
Show that you are capable of putting her desire before your own.