" You've got a girlfriend but you're still stranded on third base.'' Nothing serious was happening between the two a speaker is refering to (general things like kisses and huging).
"You've finally got to first base." That would involve a more serious and intimate relationship?
On the other hand, if you're stuck on third base, it means you haven't had intercourse yet.
I agree with Gil: I can't get to first base is the most common thing you'll hear related to this.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
It is. At first base, you kiss "and stuff." Things become progressively more intimate as you round 2nd and head to 3rd. If you have full intercourse, you've made it home.
There is a great song by Meatloaf, Paradise by the Dashboard Light, that has a wonderful commentary by an actual baseball announcer as the young man attempts to "come home." He rounds third, the commentator says "Holy cow, I think he's gonna make it..." and then the song resumes with the woman saying "Stop right there!"
I think I will listen to it now, in fact.