Something like ... Buffy, Faith, Cordelia or someone else falling asleep in something that shows off their ass... Xander (or someone else) ends up groping them and coming on it in their sleep ... feigning ignorance when/if they find out later. Can happen multiple times until they confront him about it (in whatever way they choose).
Xander fine tunes the love/lust spell to work only when he is actively thinking about and/or looking at the woman in question. That way he can fuck whomever he wants, whenever he wants it.
One of the girls loses a bet with Xander (or someone else) and decides that they'll indulge him in a handjob. He 'accidentally' comes on their face and they end up enjoying the feeling... They don't start a true relationship but they start to allow him to come on them and warn him not to waste his seed, especially when they find out how good for their skin it really is.
Plenty other weird/kinky ideas and I'm more then happy to discuss a crossover scenario if it applies!