My understanding was:
1st: kissing
2nd: breast access of any sort (over/under the clothes might be mentioned, but still 2nd base)
3rd: Genital touching with fingers
Then “all the way”
And then there was the balk. 
This instructional video may help:
During my teenage years, we had a boy that was part of our friendship group who could get a bit handsy with the ladies (he was very good looking, so got away with many liberties). His surname was Ackrington, so let’s just say that 3rd base was known as ‘an acky’.
Eg, ‘How did you get on with [random boy] last night. Did you get acky’ed?’
When I was of an age where “getting to first base” matters, Norwegian kids, at least in my area used the verb “rote” so that the expression can most easily be translated to “mess around with”. What that implied changed with group and age, and if more details were required, you’d ask.
Current slang appears to center around “hooke”, borrowed from English “hook up with”, but used in the same context.
Beyond that, no euphemisms were needed. You’d use more or less direct language. I wonder if the staying power of the baseball euphemisms in US culture is due to the prudishness of US media when it comes to sex. A Norwegian TV show about teenagers will have them use whatever words are currently in use (to the extent that the writers are up to date), including “bad words”, because sex (hardcore porn excepted) and foul language are not seen as that big deal by the rating agency, compared to violence, while the US has it the other way round. Tangentially the movie Lost in Translation was rated R in the US for sexual situations. In Norway it was rated A, for All audiences.
Oh, so that was what this talk about bases was all about! Funny, in retrospect.
If this is the correct meaning, then in my youth in the 80s in Germany:
1st: “Wir haben rumgemacht”, “we made out”
2nd: “Ich bin unter ihren Pulli gegangen.”, “I went under her sweater”
3rd: “Ich habe sie gefingert”, “I fingered her”
4th: any non-euphemistic or euphemistic, but unambiguous term for “having intercourse”, without beating around the bush.
I wonder why any teenage boy would use euphemistic language among his buddies anyway. This was the least company at that age in which sexual language would have to be censored.
I suspect that these days a lot of bragging takes place using smartphone apps.
If there is an appropriate or inappropriate emoticon out there somewhere, it will surely be found. What need for words when everyone has a camera to update their personal media collection?
Ha ha. Reading this thread and the ads were for baseball cards, baseball tickets, live streaming baseball…
Because in my experience males are a lot more circumspect in their descriptive language as opposed to females. Guys will just announce they did something. Females will critique it.
How does one have intercourse without beating around the bush?
Date someone who shaves?
Nah. Baseball metaphors for sex aren’t about American attitudes to sex so much as about American attitudes to baseball. Starting around the late 19th century, American English became absolutely saturated with baseball jargon in all sorts of contexts.
You may “get to first base” or “score a home run” on a date, but at work you can “drop the ball” or have an idea “out of left field”. An unexpected situation “throws you a curveball”, and if you can’t handle it you may need to get somebody to “pinch hit” for you. No, ascribing baseball sex metaphors to American prudishness isn’t even in the ballpark, etymologically speaking.
Maybe you’re right, but I think you are missing my point somewhat. Other cultures have persistent metaphors in the workplace and for unexpected situations too, but based on the, admittedly limited, evidence in this thread, Europeans use euphemisms much less in the context of “dating”. Of course US obsession with baseball is at the root of the first base, second base slang, and it does sort of uniquely fit, but if US media wasn’t so prudish and ensured each generation gets this particular euphemism in from an early age I’m fairly certain US teen guys would be more straightforward when kissing and telling.
Hmm, well, perhaps so.
I’m from Texas, but I watched several episodes of the Australian series Puberty Blues . I don’t recall the euphemisms for activities short of full-on intercourse, but the teens in the series call the actual act “rooting.” The actors used the word quite naturally, but I found it grating. “I rooted last night.” or “They were rooting in the back of the car.” just sounded really weird to me.
Hence the description/nickname “Wombat”: Eats roots and leaves.
Thanks! I totally didn’t get that. 
Interestingly, “bases” was recently (yes, I call 2014 “recent”) used in a Haruki Murakami short story published in the New Yorker. It would be interesting to see what was used in the original Japanese language version.
There’s always the squeeze play.