Speaking of sexual activity in terms of bases, sometimes doing so can make the interaction with your partner(s) seem more goal-oriented (i.e., to “hit that home run”, or “get to home base”).
Keep in mind that sexual activity doesn’t just have to be about how far you can get with someone.
Talking about fifth base when there's really only four bases reminds me of the expression Up To Eleven.
If you want to downvote me for using Urban Dictionary, that's fine. First base When you get to first base, you have been lucky enough to have been kissed.
Essentially, between second and third…we’ve reached the genitals. Actually, shit, amendment to First Base as well…look for cold sores. So, before you go doing, whatever it is you think you’re going to do down there…have a look around.