If you really liked Viv and had to take corruption gain every time you sexed her, and considering she's a companion which would mean lots of sex, it'd get kinda annoying for people running pure characters no?
True, but firstly from a RP perspective, if you're trying to RP a pure character you thematically shouldn't be turning her into a demon in the first place. Actions have consequences, and the game gives you like two or three chances and asks you repeatedly whether you really, really want to turn her into a demon and straight out tells you that this may not be a good thing to do. And well, if you are playing pure but turned her into a demon anyhow "by mistake", then taking the extra step of having to go to Brother Sanders and cloistering yourself and repenting that you turned your friend/lover/whatever into a soulless monster every now and then doesn't seem thematically far-fetched.
From a game balance perspective, it's also not that difficult in-game anyhow to lower corruption, given Winterstem is not that difficult to come by. It's a pretty common drop, it's randomly purchasable from the Kitsune Den, and if it's that much of a problem, I guess they could make it non-randomly purchasable from a vendor, maybe even Viv herself.
I just don't think there's a good reason for her to be an exception to the rule of "having sex with demons increases your corruption."