Given that his ideology is based to blood purity, he may very well (at least put up a show of) encouraging purebloods to have children.

Also, given what we know about Bellatrix's relationship to Voldemort, maybe Lesath is actually Voldemort's son and Rastaban adopted him after Voldemort's downfall, falsely acknowledging paternity so he wouldn't have the stigma of being the son of a dark lord.