From describing Debbie as a pet to explaining that their offspring is almost 99% Viltrumite I think the goal wasn’t to start a family, it was purely to produce Mark. I think it’s a long-term strategy. Nolan could have conquered Earth himself (maybe) but having a second loyal Viltrumite makes it that much easier. We see that in the alternate universe… He and Mark are just doing it themselves. Also why he told Mark when he was pummeling him that he can always have another kid, as in another soldier. Marks brother was probably a back up plan…