For two obvious reasons: first, there wouldn't be an incestuous relationship, and second and more importantly Yuki's friend is really ugly...
If Lizana had sex with attractive men like her husband, she’d be doing the same thing most other artists do, which would be pretty boring. It doesn’t make sense for him to do the same thing, given that his works is considerably better because of the contrast between the appearance of female characters like Lizana and the rest of the freaks she has sex with...