I've honestly never thought about doing THAT business with PROG business. Me and my former (and only) lover would just turn on the Blue album (weezer) whenever we did it. It was great fun.
I GOT IT THOUGH! (not really prog) It seems like it would be fun to shag with the rythem (and hearty mood) of "Ping Island lightning rescue Operation" from "the Life Aquatic" soundtrack. GREAT FUN! 
On a more serious note, I think it would be great to make love to "Never Comes the Day" by Moody Blues. It just has that mood. then have it skip to "Are you sitting comfortabley?" (Yes on the same album)
Yep both songs together, 8:13, thats PERFECT!!!! 
Other mentions for those longer lasting gents.
17:06 "Dogs" - Pink Floyd, yeah it would definately work.
23:31 "Echoes" - Pink Floyd, yeah Floyd's just the Prog-shag King today arn't they?
13:30 "Shine on you crazy diamond Part 1" - AGAIN Pink Floyd
20:36 "The Remembering-High the Memory" - YES! I would kill to make love to this song.Very emotional. Get a girl in bed with this song and she will remember you forever (if she apreciates good melody)
17:12 "Stories without Words" - Harmonium, Beeeautiful!
If you and your girl (or guy) are into very VERY long sessions. Try listening to the entire album "The Snow Goose" by Camel. No lyrics, just pure emotion babe! 
... oh and for those quickies... try these out
1:28 "Another Green World" (song not album) - Brian Eno
2:49 "Love Remembered" - Focus
1:47 "For Absent Friends" - Genesis, this may sound wierd to some, but at the moment my former lover would reach her climax she would begin to cry, she loved it. This song would probably have increased that. if your guy or girl is like that... 
3:31 "Hands of the Preistess Part 1" - Steve Hackett
Man, this has inspired me to call the girl that I love and admit myself to her. :D And save her from her deadbeat Boyfriend! :D...
maybe tomorrow...