[Cherry_Lover]

Originally Posted by DragoZERO

I'll agree to that. He deserved a more slow death, however he was also a victim of circumstance to some degree.

Well, he was being used as a pawn by Zouken through the whole route, yes. But, nevertheless, he chose to rape and abuse Sakura, and no matter what you might say about Shinji's circumstances, you can't possibly claim that they're even a tenth as bad as Sakura's are.

So a servant has to use sex to absorb energy unless they use a boundary field or a spell of some sort?

Well, no, most servants survive on the prana supplied by their masters. However, because Shirou is a shit magus, his connection to Saber never got established properly, so she couldn't draw prana from him, hence why she needed to get it through sex. In Rider's case, whilst Sakura can supply her with prana, the worms in her body suck out any excess prana she might have, so Sakura is always low on prana and thus has little to spare to fuel Rider.

However, sex is certainly not the only way to get prana. Other than using spells (like Caster does) to draw prana from people in the town or using Rider's boundary field to melt people, a servant can also gain prana by eating people's souls (or, seemingly, also by sucking blood, judging by what Rider does to the people she finds). It's implied that that's what Rider does under Shinji's control (and, indeed, that's probably what she did to Ayako, although Ayako survived). However, when she's under Sakura's control, eating people isn't an option (unless she does it secretly, and even then if Sakura found out she'd be very angry), so in order to reduce Sakura's burden she has sex with Shirou.