The males of those crossbreeds are all sterile and the gestation periods of deer and goats are so different, that their offspring are usually premature and die in the wild. That's what keeps the species separate. You don't need misceganation laws to stop it, as Nature already does. If you're so completely overrun with their offspring that you freak out and setup separate watering holes and territories because they're well on their way to becoming the majority of the population, because they are _more demonstrably and successfully fertile_ than the rest, then it's safe to say that they are the same species.