What are the bases dating
It occurs when your date allows kissing even though they are not attracted to you.This is called Baseball metaphors for sex There's normally no fifth base, and House was actually playing with the terms.
"It helps to focus on having fun throughout, instead of doing what it takes to reach the 'goal' of orgasm.
He explains this by saying "That's two home runs, and then she gives me back a triple." Home run is defined: Thank you for your interest in this question.
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One of the other answers has provided a nice link to the Wikipedia entry for baseball metaphors for sex.
There are, as the top comment on this question notes, four bases in baseball, and these have corresponding sex acts associated with them.