I don't mind if it's for a song or a crack pairing (LOL, Aline's hair x Gakupo, don't ask, old joke is old). I don't mind even if it's just for fanservice (like all those Ted x Kaito or Miku x Teto pics out there).
My concern is about canonicity. I'm pretty sure Luna never stated Luna x Len as canon, at least not that I know (and if she did, it was as an one-sided crush from Luna, I can go with that). But I know some fangirls who state UTAU/Vocaloid fancharacter x Vocaloid pairings as canon, without having any idea about what canon means. If your UTAUloid is being paired with a Vocaloid, even if it has thoudsands of supporters (there we go on Ted x Kaito again), it's fanon at best, because no Vocaloids have canon pairings (the nearest would be Sweet Ann and Big Al, what with that whole Bride of Frankenstein/Frankenstein Monster theme, but PowerFX never specified nor confirmed that they're a couple per se, so, it keeps itself on fanon).
Heck, even I pair Aline/Allen and Gakupo sometimes (well, 15yo Allen) for kicks and giggles. I also would love to make Kasui and Big Al interact in a bromance full of dubious undertones, but I'd never say it's canon (specially not the second situation, because ohai, Kasui is Scarfu's, not mine). So, guys, keep it cool, keep it for fun. <3