Sex Rookie is a quest you can complete to attain the status of the same name, from the starting status of Wanker. The quest becomes available once you have recruited Mary into your gang and the status is her final upgrade requirement. To accomplish it, you must buy a sex salon for rich ladies and also a fitness center for young and beautiful girls, and upgrade both to the third level. The third objective requires throwing a party (specifically a drunk sex party) and to "hook up" with a... well... we can't say that on Wikia.
If you want to play without paying money, you probably want to wait for limited time offers. The cheerleader and the upgrade of Mary will increase the combat scores of your gang, but that only means you'll fight "other players". The businesses in Los Angeles earn themselves back faster.