I'm a now bisexual but formerly straight guy whose main attraction to men is their cock. This is something I've wondered, especially when I first got into them. First, I think it's the most visual indication of arousal that humans have. Nipples get hard and pussies get wet but cocks go from soft to hard and you know he's turned on (morning wood aside of course). They can also be so masculine. Last, the shape is made for penetration. You see a hard cock and you know it wants to push into someone.

Yes, I over thought it. But they are so damn hot.