What is the origin of the phrase, "up for it"?
It looks likely to be of 19th-century origin. There are a couple of references in sense 10e of the second adverbial meaning of up in the OED.
10e. Bound for (a place); ready for (something). Cf. 19a (d).
1870 H. W. Longfellow John Endicott ii, On board the Swallow,..Up for Barbadoes.
1894 R. D. Blackmore Perlycross I. xvi. 243 Christie was quite up for it. She loved a bit of skirmish.
What were you reading that suggested a risqué interpretation? I have never considered that it was, but I can see why it might be thought to be.