I'd find it totally insulting if, after having given a guy a bj, he refused to kiss me afterwards until I'd disinfected my mouth with water, mints or whatever. It's really rude and it would seem to me like getting off is a bigger priority to him than having intimacy with another person. If it was just a meaningless encounter then I could understand, but if within the context of a relationship then it counts as an intimate moment.
If he isn't prepared to let the slightest bit of semen touch his mouth, then how on earth can he expect me to swallow the lot?