I'm not exactly sure Ataru "loves" Lum, or at least, it's shown very poorly. Of course comedy anime so any development or emotion is heightened to absurd degrees, and of course they're fated to be together by the plot itself, but watching the remake it's almost amusing how Ataru essentially has the trappings of a .... sort of domestic abuser? I'm not saying this as a critique or anything (God forbid placing such themes in UY) but the only situation where Ataru shows a minimum of worry about Lum is when she tries to dump him or some other man shows clear interest (and even then, not always). It's like "I don't care about my wife and her well-being but the instant she's going to leave me I'll go ballistic".
I understand that's probably unintended, but it looks like he gives a shit only when he's in danger of losing her (a fate that honestly he deserves), dumping on her untold amount of public humiliation and barely caring about her feelings in everyday life. The Cow episode was the first time in two seasons where Ataru had a minimal amount of empathy.
Of course, Lum is completely schizo and abusive herself, but everyone in UY is a psychotic, violent schizo and that's great.