Irl... not everyone is xx or xy. Some people are xxy or xyx. There are other combinations too. It is called "intersex" and approximately 1 in 500 people have a third chromosome. Maybe the Sims in question have three chromosomes as well? Just a thought.
Edit: I just thought, in MWS there are "intersex" wedding cake toppers. I'm sure that the genders settings are set to reflect that if you want them too.