One problem is that it's already difficult enough to get a suitable peep. To reduce the chance of failing to produce a family (or, at least, to provide a suitably large-sized 'pool' of kids from which to pick the next generation) you really need to choose a peep who "definitely" wants children. Then you need to avoid those with dislikes like running or work that would hinder their career development. The peep's career is important, too; some are very low-paid or unreliable

That doesn't include any personal feelings or prejudices a player might have: maybe you think a particular peep is ugly, or you don't like blondes, for instance. Of course, if you are looking for a Perfect Match, or even a rich (or poor) peep, things get even more complicated.