Hello, apologies for the late reply - I wanted to get the opinion of all the judges for this, and we are online at very different times.
The answer is that mpreg, while falling into the preg category, will not be allowed. This is largely due to the fact that a majority of the community we built, and by extension the judges, are not into mpreg and as such we couldn't say that we rated games containing it fairly and equally to games without mpreg. While we do not want to limit creativity and do not bash people for having this preference, allowing it wouldn't feel right since we would knowingly set up people for disappointment caused by something out of their grasp.
I hope this answer, while probably disappointing, clarifies our reasoning at least. We still hope you have a fun time with the jam!