What If Howie Hadn't Been a Virgin?

From the denouement, Lord Summerisle makes it pretty clear that Howie was "chosen" for a few essential reasons, one of them being the apparent fact that he was a virgin. What about that attempted seduction scene with Britt Ekland earlier in the film? If Howie had given in to the temptation and had sex that evening (which he damn near did), wouldn't this event have singlehandedly ruined their plans? It seems like an awfully significant risk to take given how much they had invested in the need to use him in their sacrifice.

I also wonder how it's possible that they could have known with absolute certainty that Sgt. Howie had never engaged in sexual intercourse before. Did the islanders have someone stalking him on the mainland every day since puberty?

Tangentially, I also wonder how he could have been "chosen". How could anyone have been sure that the police would send him and no one else to conduct the investigation? And then there's the peculiar fact that someone registered a complaint about the missing girl, and yet the police apparently had no idea who it was and didn't bother to verify the legitimacy of the claim before sending someone out by airplane to conduct a full investigation. Did Scotland Yard waste resources on missing persons back then every time they got letters in the mail?