All in all, the story I keep coming back to is a little FF MC story called 'Itty Bitty' on MCSTORIES.COM (at http://www.mcstories.com/IttyBitty/index.html)

It concerns a young lady who is somehow reduced to a chaste mouth-slave by a an old (female) classmate. The sense of helplessness is really something.

Not nearly hard-core enough for you real BDSMers, but for me it works.