I mean even then, its odd but I don't see a problem if modders aren't selling the mod as a "expansion" to users. Ai voices are iffy but they can bring so much cool things to the modding scene. The biggest problem is when large corporations like Rockstar/Take two/etc start using them to replace actual voice actors. A modder isn't going to have the money to pay an A list voice actor to voice act in their mod
Edit: I do think it does cross a bit of a line if said mod is a sex mod but realistically that isn't a logical difference between that and a normal mod but it does feel like its crossing a line.