Green’s Dictionary of Slang

altar of pleasure n.

[a literary euph.]

1. the vagina.

[UK]L.C. Smithers Priapeia 212: The following additional terms [for vagina] which I have culled [...] annulus, a ring, aquae fons, a fountain of water, ara voluptatis, the altar of pleasure.
[UK]Farmer & Henley Sl. and Its Analogues.

2. the anus.

[US]A. Wainhouse [trans.] Philosophy in the Bedroom (1971) 277: His buttocks will be fairer, plumper, never a hair will shade the altar of pleasure, whose interior [...] will be found positively of the same variety as the interior of a woman’s vagina.