If this was a classic Earth hero then yes, he'd be cheating. I suspect Caphans - including Kaara - wouldn't have a problem with it. After all this is a culture where Ohanna encouraged Kiivan to sleep with five of the exiled Caphans to ensure he had an heir before going to war.

Vahiir still wants Kaara as his best beloved, mother of his children, most favoured of all his slaves; that doesn't mean he doesn't expect there to be other women in his bed from time to time.

And now we're really deep into fictional cultural mores.