I figured out the reverse cowgirl thing at the
SPA training.
With Monkey installed, before intercourse (
or during, BUT most importantly before the "procedure"), press
"Y" to enable Sluffy. The bars on some strange graph on the left will rise - indicating she's ready for anything.
Well, as usual, just reach the "get on top" prompt and everything will work (
it's a shame you can't edit the penis during this process; you'll be with a random, usually "average," guy 
). Also, get Arousel 100% and some percent of pleasure and remember to tip her
P.S. I'm not sure, but your model should naturally agree to sex, although in the spa, everything clearly works because Sluffy is enabled.
I'll add that the model's "fatigue" will increase, and it seems that it can only be reduced by a week's skip (until the sign with the rental payment, etc., appears).
Good luck!