Its because they don't want to make you fell weird if they are starring at you! Don't you think it would be weird kissing and starring at each other?
There is just something unromantic about a girl with her eyes open while I am kissing them... I don't know why, but the though kind of creeps me out.
Actually it would be a strain to the eye to try and focus on a OBJECT so close. If they didn't close the eyes, they will end up in head ache and the golden moment would be lost. More over they will hold their breath also, if the lover is suffering from Bad Breath.
Guys close there eyes too.. I just think it is a natural reaction to having something that close to your face..LOL Also I would imagine that when you close your eyes, and stop using your sense of vision, the other 5 senses become more intune, including your sense of touch and the feeling of the kiss! Imagine if closed your eyes, wore nose plugs and ear plugs. Your senses's of touch and taste would be hypersensitive and the kiss might be even better.. LOL
It's all about losing yourself in the moment. Instead of being distracted by what else is going on in the room, you can completely focus on the kiss and the way the kiss makes you feel weak in the knees... LOL But you'd look so funny with the ear and nose plug that the other person would be laughing so hard that they wouldn't be able to kiss you!
Buffalo is right (as she usually is). Also, men should watch the faces they make sometimes when they are really into kissing and forget someone is there to watch.... oddness (and kinda creepy)