Should've known when I made it all the way to third base
And that was only the first date
Could've made it to home plate

from Eminem (Ft. Kendrick Lamar) – Love Game Lyrics on Rap Genius

Meaning

Baseball Metaphor

3rd base” is slang for everything except full intercourse, ie handjobs, foreplay, oral, etc.

When that all happens on the first date, he should have expected that she was a little too promiscuous. He regrets not banging (home plate) her when he could have .

Sidenote: If he’s continuing the old school Hip-Hop theme then this very well could be a reference to 3rd Bass

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