Romance and puberty are frequently featured themes and are the main plot points of several episodes.
A boy gives a merman CPR, but it is very clearly made to look like they're actually kissing.
One of the oddities in the Mystery Shack is called the "Sascrotch".
A man is married to a woodpecker, claiming that it's still "totally legal".
In an episode, Dipper accidentally lays on his crush, Wendy's, bra. She mentions, "Dude, you're laying on my bra." Dipper then screams in embarrassment.
Very frequent mentions of boyfriends and girlfriends, kissing, crushes and Mabel is 100% boy crazy throughout the whole series.
One of the main protagonists has a hopeless crush on someone much older than him. This is mentioned frequently throughout the series, and is often portrayed in a vaguely suggestive way.
One character is frequently seen wearing only boxers and an undershirt throughout, in a nonsexual context. Additionally, some characters are occasionally seen shirtless, in swimsuits or in their underwear, again, in a nonsexual context.
Puberty is mentioned throughout. In several episodes, one of the main protagonists is insecure about his masculinity deals with issues such as chest hair, voice changes, sweating issues, feeling awkward, etc.
Some characters have books or magazines with sexual content; this includes characters reading "age-inappropriate" romance novels, a book about puberty ("Carpet Diem"), adult magazines ("Scary-oke"), etc. There is no explicit sexual content in any of these instances and it is presented in a comedic manner.
There are some mild sexual jokes, including one about somebody's Internet history being creepy ("Into The Bunker"), and a brief scene of somebody taking a bath with squirrels ("Gideon Rises") etc.
One of the main protagonists (who is 12) flirts with and has brief relationships with a variety of boys, many of whom are her her age or slightly older than her, one of whom she shares a kiss with ("The Deep End").
A reference to testosterone.