It's a humorous game dealing with sexual disease, and funded by the UK's Channel 4. However the "strong sexual content" isn't XBL friendly.
"We have guidelines in place that closely track requirements of content ratings boards worldwide and, among other things, prohibit the publication of strong sexual content," Microsoft's David Dennis told Seatlle PI, reports Eurogamer.
"While we haven't seen this game, we can confirm that if it is consistent with the description we have seen on the Internet, this game would not pass peer review and would not be permitted to be distributed on Xbox Live." It's given freely on the PC.
Dan Marshall of Zombie Cow has responded to Microsoft's blush. "There was always going to be a risk it won't pass Peer Review, but obviously we'll do whatever we can to get the Xbox version out," he said. It's to promote safe sex in an "entertaining, funny way".
"It'd be a shame if a huge number of teenagers missed out on some quality gaming and vital education because of some abstract, cellular-level innards and pubic hairs." The game features marines in condom hats blasting diseases inside naughty body parts.
Privates releases for free on the PC, whereas it would cost on Xbox Live.