In a former GA mix exchange, I put together a CD called "Hittin' It" which was themed around that very topic. The CD cover was frickin' AWESOME.
Although I could probably put together about four or five CDs worth of stuff on this topic, it all basically boils down to two things: 1) Establishing some kind of mood, either lushly romantic for the "making love" crowd or intensely passionate for the "f?ck me now" crowd. 2) Regardless of the mood or type of song, the song HAS to have a consistent beat...because, you know, it helps.
So, with that in mind, here are a couple of songs/albums which come to mind:
1) Basement Jaxx, "Get Me Off." If you haven't heard this track, you SERIOUSLY need to add it to your arsenal. Holy moly stick it in the hole-y it's a kickass track.
2) If you're trying for the swanky, soothing, romantic vibe with an <AHEM> do-able beat, you can't go wrong with Naked Music. In particular, I'd recommend Beautiful Tomorrow from Blue Six, The Pearl from Aquanote, and any of the Nude Dimensions mixes.
3) One of my top ten favorite albums for MANY reasons, but making out is very high on the list: Charles Webster, Born On The 24th Of July. Seriously. Buy it. Now.
4) Some other random songs which probably don't belong on the same mixtape: Nine Inch Nails' cover of the Adam Ant classic, "Physical." Alexkid, "Love We Have." Phonique, "Thick N' Rich." Blackalicious, "Make You Feel That Way."
5) Finally, this is kind of a leftfield choice, and the CD is hard to find these days, but my all-time most successful sex album is No Protection. (No pun intended.) It's a reconstruction of the Massive Attack album Protection done by dub reggae master Mad Professor. He breaks apart each song and rebuilds them as trippy, slow, psychedelic dubby jams. The original album is ALSO a quality makeout album in its own right -- I mean, the song "Protection" is a damn classic -- but the combination of trip-hop beats and dub reggae oomph absolutely WORKS.
(The sad irony, of course, is that my GF is barely into music, and not at all during sex. <SIGH>)