Got the idea for this after watching Sailor Moon Super S episode 159 where everyone tried to figure out who Chibiusa's boyfriend is -- even Mamoru who hardly EVER does anything silly. Definitely one of my favorite Mamoru moments. I thought it was really cute how he became all father-y and protective about it...though a little more so in the English dub when he said things like, "I wanna have a talk with him" or "--But this is my only daughter!" That's one of the few things I like about the dub -- they really made the parent relationship between Mamoru/Usagi and Chibiusa a lot cuter than the original, like for example, with Chibiusa calling Mamoru "daddy-kins" and him calling her "pumpkin". It's adorable.
Anyways, with that thought, I figured a protective dad like him would take this sort of approach if he did ever meet Helios as her boyfriend in some...alternate universe where he could live with normal people. Yeah, I imagine Mamoru wouldn't be thrilled. (And he doesn't even know that Helios has kissed his daughter several times, hung out every night in her room throughout the Super S series, and even saw her shirtless once!)