What’s the best euphimism for sex you’ve ever heard? My personal favorite (and one that pisses off the girlfriend) is “throwing it in,” as in this example:
“Hey, Kev, did you throw it in [girlfriend] this weekend?”
Thanks to Frank Zappa - Plook
This activity actually is most commonly implemented with a gleaming model XQJ-37 nuclear powered Pan-Sexual Roto-Plooker and a Gay Bob Doll (he goes all the way). But it’s best if you speak German.
I’ve always been partial to PORKING.
My ever romantic husband is partial to:
Slappin one up in ya
and
Stickin’ it in and movin’ around a bit
and Jack Batty,
isn’t it always? 
jarbaby
I don’t particularly like any of them. But “Making the Beast with Two Backs” sounds literary, and I think it’s in Shakespeare.
And in one of my high school English Lit classes somebody suggested that “the beast with two backs” was a camel.
“Chucking a sausage up the Mersey tunnel” is a favourite.
Not a completely generic expression though. It refers to when (how do I put it?) the sizes of the participant’s bits are significantly at variance in favour of the female.
“Taking the skin boat to TunaTown” ? That was a good one from the film ‘Grumpy Old Men’, IIRC.
For a gay friend, another friend pointed to a rather camp male stranger and said that he looked as if he was in the habit of ‘receiving swollen goods’. Best pun I’ve heard in years, and he claims to have invented it on the spot.
• Laying pipe
• Doing the horizontal mambo
“I’d walk a mile for a vertical smile”
CalMeacham, that’s one of my favorites as well.
Othello (1605): Iago: I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with two backs.
Really, anything preceded by an “um” will do. My summertime favorite: “We were…umm…checking each other for ticks.”
My husband and I tend to use “playing Parcheesi”.
I always liked “knockin’ boots.”
Not excactly sure which part of the body “boots” refers to, but I am inclined to think it means the plural of “booty.”
And my other favorite has always been “doin’ the nasty.”
Other memorable ones I’ve run across:
[li] Ridin’ the pony[/li][li] Greasin’ the squeaky wheel[/li][li] Plowin’ the south forty[/li]
Oh, and one other thing - if the expression includes a present participle ending in “ing”, and they nearly all do, then the proper way to spell them is with an abbrieviated “in’”. Thus knocking becomes knockin’, and doing becomes doin’.
Saying the expression aloud and including the ‘g’ sound at the end of the participle just sounds wrong.
I was talking with a co-worker about an encounter with the guy that I eventually married. I was explaining why I was so tired that day. I was getting to the good part and was saying ‘so we spent all night…’ then I noticed my Baptist boss was standing right behind me. So I finished the sentence ‘…reading Bible passages to each other. And we fell asleep. On the couch. Fully clothed.’ My boss laughed her ass off, because she does have a sense of humor.
So, we use ‘reading Bible passages to each other.’
Oh, also ‘boffing.’
bughunter, I always thought “knockin’ boots” referred to actual boots. Maybe not, though. Somehow your suggestion of booty seems off to me, though.
I was greatly amused the first time I heard “bumpin’ uglies.”
I have always been partial to shtuping (sp?). I heard Bette Midler use it, and it just kind of stuck.
Jeeves
One of our (Mrs. B and me) favorites is also from Grumpy Old Men; “riding the baloney pony.”
Or, to put it a wee bit more politely, “Nik-nik”
(Latka from Taxi)
Oops, Bughunter already got “riding the pony”
Sorry about that. I was reading too fast, and I…skidded across that post, yeah, that’s it.
“Doin’ the nasty” or “doin’ the wild thang.”
The best part of GOM was the outtakes with Burgess Meredith at the end. In addition to the above, and “…boloney pony”, there was “takin’ ol’ One-Eye to the optometrist”.
Referred in my house as “Telecommuting”, because that’s the best part about working at home. 
RATS! beagledave beat me to “horizontal mambo”!!
“Boinking”, which ahem requires the final “ng”. “Boinkin’” just wouldn’t cut it.
I have my own personal reference for said act.
Rocking The Spank Shack
This of course applies also to anything else deemed really cool, or spank-o-licious in my book. And of course anything yummy derived from BDSM pleasures.
i.e.
Those shoes rock the fcking spank shack!
or
His C#@* rocked my spank shack… yada yada yada.
I know, I know - Personal Ad Libbed Connotations Are Never as funny as ones heard for 80 years or so. But i think i’ll have that engraved on my tombstone anyway. Something to the pathetic effect of…
“Here lies Azura… She rocked her own spank shack”
sigh
