In "The One With Rachel's Inadvertent Kiss", after getting a strong handshake from Monica, Phoebe remarks that she's glad she's not Chandler (Monica's boyfriend).
One episode discusses fore/afterplay, specifically the importance of keeping the intimacy going.
Chandler: Yeah, I think kissing is, for us, like an opening act. You know, like the comedian you have to sit through before Pink Floyd comes out.
There was also the time the friends practically read aloud an extremely explicit pornographic story rendered harmless by Rachel's mad computer skillz:
Rachel: Okay. Now this is just the first chapter, and I want your absolute honest opinion. Oh, oh, and on page two, he's not 'reaching for her heaving beasts'.
Monica: What's a 'niffle'? Joey: You usually find them on the 'heaving beasts'. Rachel: Alright, alright, so I'm not a great typist... Ross: Wait, did you get to the part about his 'huge throbbing pens'? Tell ya, you don't wanna be around when he starts writing with those!
And the time that Rachel went on an independence kick after reading a heavily metaphorical book about female self-realization (which referred to not letting men "steal your wind", i.e., independence/power.) She eventually has a fight with Ross about this which is hilariously misinterpreted:
Ross: Your...wind? Rachel: Yes Ross, my wind. How do expect me to grow if you won't let me blow? Ross (nervous): Oh...Y-you know I-I don't, have a - have a problem with that.
When Monica tries buying and selling stocks:
Monica: Get out before they go down, that's my motto. Joey: That is so not my motto.
Alluding to the less mainstream sexual acts when Rachel mentions to Ross that they never had a "bonus night" after they ended their relationship:
Rachel: Hey, you know what we never did? [Ross looks suspicious and intrigued at the same time] Rachel: No, not that!
In one episode Chandler spent all day playing Pac Man and putting in dirty words for the high scores screen. The words are never shown, but they are commented on.
Monica: That one's not dirty. Chandler: It is when you combine it with that one. Monica: Well, if you don't erase this, you won't be getting one of those from me!
When Rachel was helping Ross move his new couch.
Rachel: Chandler, can I borrow your tape measure? Chandler: It's in the bedroom. Monica: Yeah. (she and Chandler start giggling)
In one episode Joey is participating in a fertility study and can't sleep with his girlfriend, who is expecting them to. When he asks Monica what to do when they get intimate, she says to "be there for her."
In "TOW Ross Gets High", the gang are wondering how Rachel managed to botch a recipe so badly.
Ross: The pages are stuck together! Joey: (accusingly) Chandler!
In "TOW Ross and Rachel...You Know", when Ross and Rachel finally...you know, they both feel something wet and Rachel assures him it's OK. Ross is quick to tell her she only rolled over a juice box.
Monica and Rachel offer to kiss each other in front of Joey and Chandler to win their apartment back in "The One With All The Haste". It works.
In "The One With Rachel's Other Sister", when Rachel got into a Cat Fight with her sister, Amy:
Phoebe: Oh my god! Shouldn't we stop this?
Joey: What? Are you out of your mind? Lets throw some Jell-O on them!
In "The One With The Birth Mother", Phoebe and Rachel discuss helping each other try on bras, and Phoebe says "Wait a minute. Are we in Joey's imagination?"
In the "What If" episode, "The One that Could Have Been", Ross is still married to Carol. She suggests a threesome in order to spice up their sex life. Ross later confesses to Joey that he was mostly a passive participant, and he even made a sandwich while watching. However, Ross mentions to Joey that he got a little bored while watching (which is the reason he made the sandwich), in which case that'd make this an aversion. An example of "Girl On Girl ISN'T Hot".
In "The One with George Stephanopoulos", when the guys want to go to a hockey game:
Ross: You know what, I'd better pass on the game. I think I'm just gonna go home and think about my ex-wife and her lesbian lover.
Joey: The hell with hockey, let's all do that!
In "The One with Joey's Big Break", Rachel is afraid to take her eye drops, and Monica tries to force her, and ends up straddling her and pinning her arms down. Rachel says: "You know, if Joey and Chandler walked in right now, we could make a fortune!"
In "The One with the Chick and the Duck", Monica slams into Rachel when she practices roller skating, knocks and falls on top of her. When Joey sees them, with Rachel moaning (in pain), he says: "Oh, my!"
In "The One with the Flashback", when Janice asks if anyone of the group ever slept together:
Joey: Well, there was that one time that Monica and Rachel got together.
Monica and Rachel: What?!!
Rachel: Excuse me, there was no time!
Joey: Okay, but letís say there was. How might that go?
In "The One With The Nap Partners", Phoebe, in a desperate attempt to win the judges' vote for maid of honor, makes a toast regarding her and Rachel:
Phoebe: I can't believe that Monica and Chandler are getting married. I remember talking about this day with Rachel, while we were showering together... naked.
Joey: And she's back in the game.
There's also the time that Monica drew an (unseen) diagram of the female erogenous zones and numbered them all, then began to describe what order they should be touched by referring to the numbers:
Made even funnier when, later in the episode, Chandler's girlfriend rushed in in a pretty dishevelled state, hugged Monica whilst yelling 'THANK YOU!', and then ran back into Chandler's apartment.
Also, Chandler doesn't initially recognize an erogenous zone, but then says it's because he was looking at it upside down. Rachel remarks, "Well, you know, sometimes that helps!"
When Ross has an allergic reaction and his tongue swells up, Joey is the only one who can understand what he's saying:
Joey: My uncle Sal has a really big tongue. Chandler: Is he the one with the beautiful wife? (Joey nods)
Ross keeps annoying his ex-wife Carol on her anniversary with her new wife, Susan. When she comes to the door, she is seen to subtly pull something away from her mouth, and retorts "Uh, no!" when Ross asks if she had been sleeping. Earlier in the episode, when she mentions it's their anniversary, Ross is confused, as their wedding wasn't on this day. She fumbles and says, "it's for another anniversary. . ." Her voice trails off as Ross realizes what she's talking about.
After Chandler has swiped Monica's bran muffin and Phoebe is talking about her new boyfriend, who's in the Navy and has just returned from submarine duty:
Rachel: So wait, this guy goes down for like two years at a time? (Chandler groans in frustration, as his mouth is stuffed with food and he's unable to make a joke) Monica: That'll teach you to lick my muffin. (An even more frustrated reaction from Chandler)
In "The One with Joey's Bag":
Monica: (entering to Central Perk) Guys! Guys! I just saw two people having sex in a car right outside. Ross: Uh, Pheebs' grandmother just died. Monica: Oh my God, I'm so sorry. Phoebe: It's okay. Actually y'know what, it's kinda cool. 'Cause it's like y'know, one life ends and another begins. Monica: Not the way they're doing it.
When Monica is depressed over breaking up with Richard, Ross tells her she needs some sleep. Monica says, "I need some Richard."
When Monica and Chandler are planning to adopt and they meet Erica, she says she doesn't know who the biological father of her child is, but it's definitely one of two guys, one of whom killed his father with a shovel (causing Chandler to dub him "Shovelly Joe"). At the end, however, Monica figures it out:
Monica: Well, it turns out that Erica didn't pay much attention in Sex Ed class, because the thing she did with that prison guy...it'd be pretty hard to make a baby that way. Chandler: Oh God! What was it? The thing that we hardly ever do or the thing we never do? Monica: The thing we never do. Chandler:(nodding approvingly) Shovelly Joe!
On "The One With the Tea Leaves", Phoebe goes on a date with a guy that she met right after reading in her tea leaves that she will meet a wonderful man. Unfortunately the guy turns out to be a pervert.