And if Rider outright asks for it...?
I'm admittedly not sure where it would go, but Rider would be doing it because she needs prana, and sex is a source of prana, so I could definitely see the discussion heading in that direction.
Well, then Rider might find herself in debt to a Faerie. Which can be sort of unpleasant. 
It just depends on how Wynn collects.
Well, yeah, that would be a potential issue, because Rider isn't going to accept anything that could cause her to have to harm Sakura or family, or to not be able to reliably protect them.
However, Wynn's not going to turn down sex with a semi-divine being where she can dom the shit out of the other party.
Lol, indeed.
Hey Brah, sex can transfer prana. Tantric Rites and all of that. Wynn would pick up pretty quickly how to open up a connection to Miss Medusa. (Which, Rider is going to be in for a treat because Faerie prana is damned powerful.)
Yeah, exactly.
Then how is Sakura supplying prana to Shirou...?
The same way Rin does.
Except that Rin gives Shirou prana by forming a contract with him, which only requires her to have sex once. Sakura is clearly shown to need to have sex with Shirou on a regular basis in order to dump prana into him.
So, no, it's not done the same way as Rin. Don't ask me
why that is, or how it works, but Sakura definitely does not do the same thing Rin did.
