This is usually announced or thought of after seeing a girl who is distractingly attractive. The act of explaining you like the looks of a girl enough to masturbate too.
See, she's hot dude I'm going to ask her out.
Yea I'm masturbating in my head to her.
I fucking hate you.
I'm still masturbating in my head to her. Still not done, not done, Alright I'm done. I'm going to go make a sandwich. Good Mental Masturbation. Actually, you want to go to Chipotle?
Not with you.