He said it rudely, but he has a point. If people are going to get upset about having humans fuck with women who were previously fucked by dogs, it seems reasonable the reverse is also true. (see Afterparty reactions)
For the sake of avoiding argument, keeping the two fetishes and the women taking part in them separate would be for the best. If you want incest, target it at a character who is already doing incest; if you want beast, target it at a character who is doing beast. Mixing the two is going to ruin it for some people, and it also makes it complicated to work into a timeline and justify it, which Jack seems to be trying to do.
To put that into practice: if the next video involving the mom has the son fuck her again and the horse has been forgotten about, how would you feel? You know that can happen, because the monthlies are decided by vote.