FACT: I am a guy. Now that that's out of the way, please also note that I work out, and my gym (being a university facility) is open to male and female participants. Now, most of the time, the weight section of the gym is kind of a boys' club. Most especially when you see the bodybuilders in there, and most particularly in the free weight section. The cardio area, 50 feet away, is primarily (but neither exclusively nor overwhelmingly) female, and the weight machine section in between is about 50:50. I started working out during the day because it's less crowded. More alumni and faculty work out during the day, meaning I'm more likely to connect with my fellow coworkouters than when it's heavily populated with undergrad frat boys doing the Spring Break Bicep, Pec, and Maybe Abs workout.
Then things got complicated. The Kinesiology Department's weight lifting classes (yes, it's an actual one-hour class like bowling, walking, tennis, etc.) take place during the day. This alone is capable of tripling the amount of females in the weight room (not to mention they're all undergrads, see above).
Sometimes, I need to concentrate when I'm doing a particular exercise. Easiest way to do that is to pick a spot on the wall that's directly in your line of sight, without turning your head, as you sit/stand/lean at whatever machine, bench or exercise you're on, and concentrate there. Things get confusing when you're moving while exercising, such as doing squats or lunges (can you tell today was Leg Day? Yeah, I decided I didn't like walking normally).
So, after you're good and psyched up to do your particular exercise, I've found that the difficulty of the exercise is directly proportional to the hotness of the chick walking directly in front of you. I call this the Brewer Principle, because I can. Under normal circumstances, you'd care little for what observers think about the weight you're lifting and just lift it; under the Brewer Principle, you feel like less of a man if you don't lift it all. Also under normal circumstances, you care little about what noises you make (these can come from either end, and I'll leave it at that); under the Brewer Principle, you can't use the little adrenaline rush that comes from voracious grunting to help you through those last few reps because you want to show off by doing the exercise quietly, while at the same time you don't want to look like you're showing off by grunting.
See? Men really are as complicated as women.
Suffice to say, I try to do whatever exercise with consideration for anyone who might be directly in my line of sight whilst doing said exercise. At one point, I waited several minutes because I was on a machine facing directly at the back of a female coworkouter and wanted her to finish instead of thinking I was staring at her butt while exercising. Sure enough, as soon as she got done, another chick took her place. Later on, while doing lunges (I typed lunches three times before getting it right there), the same chick was directly in my line of sight again, but her face was so only when I was down on my knee. My excuse? I was looking directly behind her, where the window panes make a Cross. Kinda neat how they do that. I can focus on the Cross while working out.
But she got stared at because she insisted on getting in my line of sight.