[Moczo]

Originally Posted by DragoZERO

Yeah, but sex is not the only way, right? It's just the easiest. I think Rin explained it in Fate before she kissed Shirou.

Originally Posted by Cherry_Lover

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.

Blood and sex have been shown at least once to be roughly the same thing in the Nasuverse, over in Kagetsu Tohya. Either one can be used to transfer Prana between two individuals. Transfer by blood is quicker and easier, but sex works better and allows the donor to give more energy overall with less cost to themselves. Rider most likely sucks blood for that reason; it doesn't work as well, but she can just bite down and suck their soul out in a few minutes, then move onto the next. Quantity over quality.