Writing the 'Naughtier Side' of Fanfiction

A (hopefully not silly) question for those who include smut in their stories: can smut ever be 'out of character' for a given protagonist?

I've seen a few things/memes cropping up on my dash on Tumblr over the last while - namely that once a reader feels the smut they are reading is OOC for a particular character they are invested in, they will lose interest.

How much of that is subjective though?

If in the game, or in any other media of course, you never see the character participate in saucier activities, how can you truly know what might be 'in character' for them? Do people just infer that aspect of their lives from the personality etc that you are exposed to in the game/media? And therefore, if the fic piece doesn't fit their own headcanon for what that person would be like in that situation, they would consider it wrong/OOC?

As I said, I hope this isn't a silly question, and I hope I've explained the question adequately! I would be interested to hear if anyone has any thoughts regarding the above.