Any excuse for me to think about Proto-Indo-European roots and sex at the same time, right? I mean, we could get some number theory in there and we'd have everything I loved about college going on at once.
Some suggestions:
conjoice, conjoicing - the co- prefix plus the joi root from enjoy and rejoice. Gets extra flavor from its closeness to conjoin. Benefits: uses roots everyone knows, good Latin pedigree, definitely intransitive, has the clear sense of with-ness, sees sex as inhernetly celebratory. Issues: not obviously sexual without the extra flavor, and thus potentially euphemistic.
conamore, conamoring - suggested by
awbryan. A fairly literal rendition of "loving-with," and possibly overemphasizes the emotional aspect at the expense of the physical, but much better than "making love" as it makes the loving the active verb.
gether, gethering - Using the Germanic root from together with the prefix stripped. Literally, "joining," with the potential extension "joining in a [single] body" - Shakespeare's 'beast with two backs'. Emphasizes the physical act, possibly too much, as the base PEI root means "fitting together" like a peg does in a hole. Having said that, I should admit that I prefer the 'earthier' sound of the Germanic-derived words to the 'scholarly' sound of the Latinate ones for sex. However, as has already been pointed out, English happily gathers words from old Anglo-Saxon, Middle French, and Latin or Greek, so as to have three levels of formality for the same idea.
I want to try something using the *ka- root (descendants: whore, caress, charity, the Kama in Kama Sutra) but I can't get it to come out right. Similarly for *legh-, "to lie," because the correct formation ends up too close to "college."