I don't think Tarkatans do mate. Mileena is not necessarily a Tarkatan, but more of a creation of sorcery, a clone of Kitana. I think all we know is that Tarkatans are mutated hybrids of Netherealm and Outworld species.

They might mate, but it probably not necesary since there Pretty much Demon like. and for Mileena She was a clone of Kitana using Tarkatan DNA and Sorcerey Just to keep her looking like more of a human. It was once rumored that Shao Kahn was part Tarkaten but that has never been proven.

In Jade's ending (Deception) she throws Tarkatan escence onto Tanya. They percieve her as "a rival male" and work themselves into a frenzy. Just about every animal known that fight 'rival males' are fighting for the right to mate, or for land, but the fact that theyare considered male would mean they have to be able to mate, as they would have male reproductive organs, and if there are male reproductive organs then there are female reproductive organs. They just likely remain at home while the men go into battle, or maybe the women are that damn butt ugly they look almost identical to the males (Mileena has human features, saving her from the butt uglyness)

I remember in Konquest mode Baraka explains to Shujinko that the Tarkata are a half demon race. They are a mutant breed.