my eyes are generally drawn to cleavage. Even if it is gross cleavage (e.g. I was at walmart yesterday and an extremely obese lady, unattractive lady had her boobs half out with star tattoos across the top of them) our eyes are drawn to them... Definitely doesn't mean we are attracted to the woman or the boobs but we will generally look
Other people have a right to privacy, and it would objectify a girl's body if I were to stare. So I try not to look. It also depends on her facial features and her hair, etc. But I wouldn't date a girl that showed cleavage, because I'm looking for a girl that is modest and doesn't wear revealing clothes.
Good cleavage catches my eyes. After looking at it for a few seconds, I turn to other parts of her body, and determine whether she is hot or not all together. If she is, I look at her entire body (not just her cleavage), if she's not, I look away.
"Man, her clothes are quite revealing - I'm certain showing off that much cleavage is intentional"
yeah I things its normal for a guy to check out a girls clevage if she has nice boobs , if he's not checking them out you might want to wonder if he even likes girls
I know my eyes always drift there or glance there.
As for what goes through my head - nothing really. It's just like filing away another piece of information, like the color of her hair or her height or whatever. "This is Marie, she's blond, cute face, nice teeth, about my height, her t*ts look like that,..."