I've been on a shoujo reading streak recently, and this is something I noticed: often, when the heroine has some sort of supernatural bodyguard or guardian or assistant, she has to "feed him magic/energy/power" through a kiss (or implied sexual contact).
I wonder if it developed from the vampire idea (paying in blood)... or maybe it's just a way to "justify" lots of kisses? ^^

I think it came from Type-Moon.

In Tsukihime and Fate/Stay Night, mana is transferred through sexual intercourse.

These shoujos you have read are merely tamer versions of it.