Will I. Be wrong if I write a dude that I use to mess with when I got a man and we live together

You know the answer is the same answer it would be if the man you are living with wants to write a female inmate he used to mess with. That being said, we do not know how deep your feelings are/were and what your reason for writing truly is. If you just want to make them feel good and send them some things to read, maybe some pictures, then you know what you are doing... Keep us in the loop as to what you decide to do...