Personally I would've rated Bayonetta a 15. Although there is a lot of implied sexual themes, no fully explicit nudity and a 15 year old should be able to handle that if they can legally get married and have kids a year later; there is gore but only in the context of defeating and killing enemies which are arguably not very humanoid and portrayed as otherworldly creatures. Couple of naughty words which again every 15 year old will have heard before. And although themes involving devil worship or inciting demons used to be a big deal back in the 70s/80s I don't think it is any more.
I'd say it's closer to 15 for content and context than most of the games that are rated 18 are (often for being more realistically imitable with gang involvement, modern day assassination or stealing, drug use, murder, etc.)
In fact yeah, I'd say if a game is quite sufficiently removed from being imitable in real life, for having overblown magical or supernatural powers or elements it is less likely for someone to attempt to imitate it, so a lower rating is probably fine.
