I was listening to my sister, who is married, talking with one of her female friends about relations. At one point, my sister said, you're married, not dead.
Meaning, I took from this, that it's normal to be attracted to other people, and to have fantasies of all sorts, some wild. Now, actions and how and whether you choose to act on these is another matter.
I think trying to control one's thoughts and/or fantasies is a trap and counterproductive. In particular , during masturbation I think one should feel free to let fantasies roam and explore (in fact trying not to think of something is almost a sure-fire of making it more exciting!). The books by Nancy Friday such as My Secret Garden are about sexual fantasies women have, including during sex with a partner. And I guess I think that's okay, too. Or, if thoughts are to be controlled, more in a light, kind of zen way.
Okay, I personally need a lot of alone time, even during a relationship. I need my walks, my writing, my individual outings, to do my own thing, to feel emotionally centered, to emotionally process, to feel like I'm myself.