Mostly often is wrong!!!! Most often means you do it a lot and do it more than any other thing. In this situation, men think about sex more than any other subject. Mostly is similar, but we use it more for everyday activities and things you do most or the majority of the time.

Most often. Most is used to describe the amount of something such as "I ate most of my meal" whereas mostly is used to talk about degree or frequency (don't kmow how else to say it) such as the concert was mostly men with a only few women.