American sometimes use baseball as a metaphor for sex, because baseball is a beloved traditional sport in the US, and the metaphor will be easily understood. Leaving out many details, the goal of the game of baseball is to score "runs" (points). In order to score a run, a player must advance from "home" to "first base"; then from "first base" to "second base"; then from "second base" to "third base"; and then from "third base" back to "home". When you reach home, you have scored a "run". With regard to sex (and leaving out all details), if a person has a "little bit" of sex, we say he got to first base. If he did something more, he got to second base etc. etc. You can read about anything on the internet these days: http://en.wikipedia.org/wiki/Baseball_metaphors_for_sex

I think that should be obvious.

Bases were used when I was in high school (16 years old) but not really after that. More of an immature view of sex. I think it is just a way for young people to brag or discuss sex without using more specific terms. Like I said, not used much after those first years of relationships, egos, experimentation, nervousness etc.

I need more context, but many things are used as metaphors for sex. Sports (like baseball) are commonly used. If you are referring to the 4 bases in baseball, that is another discussion but still falls along the same lines of using sports or competitive events as metaphors for sex.