[bLike the werefox.
Your idea on multiple partners works, although if I may add an addendum:
If the subject of multiple attentions rolls low enough to fail three or more saves, s/he moves two steps up the scale unless a secondary Will (straight) save is made at DC 15 + 1 per partner.
EDIT: Example:
Lirona, the houri, has managed to attract the attentions of three of the kings best concubines and is currently having a "session" with them. The save DCs for each are 12, 16, and 18. If Lirona rolls a 12, she only goes from aroused to peaked....but if she rolls a 10. Still, she has her will save...and rolls a natural 1.
She becomes overstimulated. The concubines, of course, had this planned, making the girl a new gift for their master. When she wakes up after the long session she finds herself in concubine's garb, and it's very hard for her to remember why she agreed to join the harem in the first place . . . .