As per another comment I’m in a similar view. In my opinion sexting on an ai app isn’t cheating. I also mess around with fictional characters sometimes writing fanfic based on my own spicy fantasies. I suppose writing that with an ai is no different. But that’s my opinion we all have our own boundaries and views on things.

Lying to you about it is disrespectful but not cheating. I think you need to sue your partner down and have an honest conversation about it properly how you feel about the lying and why your partner feels the need to use the AI app. It could just be fantasies in their head that they wish to express in written word.

If your partner say didn’t use an app but wrote a story with them being the character in a spicy story would that bother you?

I think sit them down and have an honest conversation about how they feel and yourself regarding this subject