If you need a justification you can just assume that they all have an IUD. They're quite popular these days and once inserted will provide continual birth control for somewhere between 3-12 years depending on the specific type and brand used. No pills, no condoms, just totally non-visible birth control with no maintenance or anything. And, again, at present they're common enough that it wouldn't be completely unrealistic for an entire group of a dozen or so women to all be using them.
I actually considered this. And I figured that, sure,
some of them might be wearing one. But all 13? Kinda felt like pushing it a bit. But, eh, it's a plot that already hangs on a convenience after convenience, so whatever.
Or look to the source. There's no reason why our MC might not have had a vasectomy.
Or dude's just firing blanks. I can't imagine smeat is good for anyone's health.
Considered this, too. But you guys should consider the narrative implications, as they're pretty damn squicky.
I mean, the girls need to somehow be aware of his condition, considering they're the ones who initiate all the sex. And... how did that happen? After all, it's probably not something a guy would just go around and
advertise, would it? Did Jack simply drop it in casual conversation whenever he could while on the boat? "Here's your martini, miss. And in case you're interested in some unprotected sex, I'll have you know that I'm totally shootin' blanks."
And if one of the girls somehow found out, spreading that information to the entire group, especially without the guy's consent, would be a pretty freakin' scummy thing to do. And the devs are obviously trying to make the entire cast of characters, from the MC to the harem, be well-rounded and likeable. (Well, Karen's an exception.)
Yeah. Out of these two, I think I'll roll with the IUDs.
The bigger question is: who's the hairdresser? Everyone keeps their hair cut and styled and no one's pubes look like a jungle after several months of growth.
While the question was evidently tongue-in-cheek, the obvious answer is the storeroom stocking enough personal grooming implements for a small army.