According to researchers, testosterone levels increased in men when attractive women were present. Since testosterone is a sex hormone, and has also been shown to play a role in fight-or-flight responses and risk-taking behaviors, this result is not surprising. Is anyone shocked to find that men subconsciously try and "be more manly" in front of pretty females? I filed this study away with the other blatantly obvious papers under the heading "Well, no kidding."
In fact, this little animation that came out on the web last year titled, "What Would Penis Do?" sums all of this up very nicely. To save myself some time explaining it with text, I will instead embed the video, and let the narrator tell the eloquent story about man's battle between the heart, the mind, and the penis.
*note* Viewer discretion is advised. This video is PG-13, so take caution before clicking if you think you might be offended.