If we say home base = fourth base, that's 4 4 − 3 = 5, the subtraction because he was receiving instead of giving.
But this is a joke, and probably not intended to be analyzed too closely. Although this wasn't what "House" referred to, Urban Dictionary lists fifth base as anal sex.
If you have any urgent questions about the spam, please contact us.
And a special note to you, spammers; we officially declare you a major DON' T.
Asking for consent before, during, and after any sexual activity can help ensure you've expressed what you want, you know what your partner wants, and that consent has been given.
To use the expression correctly, you get or make it to a base, per my comment: quotation, if I interpret it correctly, they had intravaginal sex twice and then she performed oral sex on him.
There are, as the top comment on this question notes, four bases in baseball, and these have corresponding sex acts associated with them.
"I got to second base with that girl yesterday." "I didn't think I had a chance, but I made it to third base with her last night." One of the other answers has provided a nice link to the Wikipedia entry for baseball metaphors for sex.
Why kissing is deemed so racy that it actually needs a sports metaphor is kind of beyond me, but I'll go with it. Third Base: Generally speaking, reaching third is all about hands in the pants. To me, oral sex deserves its own base way more than a measly grab.
Second Base: Rounding second involves copping a feel. If it were up to me, I'd include the whole feeling up business in first base.