Really? Ritualized sex and magic have been tied together in human culture for a while. Using it as a transfer of energy isn't even that much of a foreign concept.

After all, the idea of people having their "energy" or "soul" sucked out of them through sex is a fairly common concept (see succubus). So using sex as a willing transfer of energy isn't a that big of a leap from there.

Okay, 1, that is not how erections work. 2, having looked at the source for this, that has got to be that most convoluted reason I've ever seen for two characters to have 'sexy times' and that's saying something as I have both watched and read the novelization of Wicked City.