It’s part of his orders. One of the things he was required to do for the Viltrum Empire was to reproduce with a local to create a Viltrumite hybrid that could help them take over the planet.

Also, their relationship is more complicated than that. Yes Nolan does have sex with Debbie, but he also does care about her in a general sense, viewing her as a subservient human who he can talk to and have sex with rather than as an equal