Many sexual references during the show, such as those to sexual body parts.
There are some episodes where the women/girls are featured in bikinis.
In several episodes, especially the season 4 episode "Class Picture", Donna (or others) are seen disrobing, though, nothing below the neck is shown/nothing below the bellybutton is shown.
The gang makes reference to intercourse countless times in pretty much every episode and refer to it as "doing it, do it, done it, or did it"
There is an episode where the gang goes skinny dipping but nothing is shown below the shoulders
Fez is usually the one making the more explicit sexual jokes.
Fez makes a couple overly explicit references to masturbation. The most crude jokes are made in the 4th and 5th seasons. (Surprisingly explicit for a TV-14 rated show)
The first time that Eric and Donna "do it", Donna's bare back is shown
Throughout the series, most of the characters are in relationships and are seen together in suggestive actions
Kitty is a nurse, so she usually uses mildly explicit references to body parts (uterus, vagina, scrotum)
Kitty goes through menopause and it is referenced countless times over the course of the season. Red and Eric are unsure about what exactly menopause is, so they look it up and read the description aloud.
Kitty and Red are very obviously shown to still have frequent sexual intercourse.
Fez's character is said to have sexual needs
"Girl on girl action" is referenced many times by Hyde and Kelso
There are a lot of of sexual fantasies
In one episode, Eric points out that a girl might be sliding all over a guy moaning like a ghost.
The words "banged" and "nailed" are used as reference to sex
There is one episode in which the central plot is that Kelso couldn't get an erection while making out with a girl. Although the word "erection" isn't actually used, the characters make pretty explicit jokes and the audience gets the idea.
There is another episode where the central plot is that Donna walks in on Eric masturbating. Although the word 'masturbating' is never said, the characters all make somewhat explicit references and jokes toward it.
There is a running joke that runs through a couple seasons about Kelso's infidelity and how he cheats on Jackie with Eric's sister.
Eric's sister Laurie is an extremely obvious depiction of an extensively promiscuous girl. She is called a whore, slut, tramp, the devil, bitch, skank, and many other names by Eric's friends.
A recurring, secondary character is depicted as a hippie who lost his mind during the war and is now a marijuana addict. He says very many sexual innuendos without realizing it while the rest of the characters see the innuendo, and on two occasions, unknowingly makes strong crude references to masturbation, which are surprisingly explicit for a TV-14 show.
Donna and Eric are seen kissing while on a bed with one on top of the other. This kind of scene is shown countless times.
Overall, the sexual content in this series is mostly on a PG-13 type scale, but during the 4th and 5th season, it seems to reach an R type level before it tames out again, just to give you an idea.
In an mid-first season episode "Eric's Buddy", Eric is kissed by a guy (guest star Joseph Gordon-Levitt).