It's kind of complicated and tight, is the thing, for a poorly-tested process, he isn't sure that he can detect whether or not a vagina contains semen in the first place... but then it's basically backup against an itself-improbable conspiracy, not the primary means of birth control?
One also needs to consider that, at a sufficient level of magical power and ill intent, they could have done something clever when he had sex with Carissa the previous day, quickly moving semen from her mouth to her vagina, maybe as screened by an illusion. Heck, they could've teleported sperm right out of his epididymis while he was sleeping in the villa the first night, before the Forbiddance went up.
Keltham lets out a sigh. "No promises, but I'll think about it, and tell you tonight whether to prepare Alter Self tomorrow. I'll also think about what a consensual demonstration of benevolently unused mind control capability should look like. And I guess I should now ask, having come up with my own insane method, is there some very standard thing that male fourth-circle clerics do, to not have kids?"