Manaka is getting tired of not getting any action, and decides she wants a sextoy. Not one of those electric ones either, she's a magi. It needs to run on magic or be alive to be worthy of her. Shirou's in good shape, attractive, and so stupid that no one will noticed when he's magically lobotomized. Plus, he already hangs out at their place a lot.
Perfect.
Oh yeah, and he's not a magi, so he doesn't actually count as a person. Even better!