Bike helmets are good things; you should wear one. I have a whole diatribe about how NOT wearing a bike helmet is selfish, but this isn’t really the place for it.
1.) Bike helmets help protect their wearers massive brain damage. I don’t find adults in diapers and unable to control their drool very sexy. Most folks strive for me to find them sexy.
2.) Conversely, if you happen to have a date that you sort of regret agreeing to, but feel morally obligated to attend, a bike helmet can make you less traditionally sexy. The basic and subtle way would be to bike to the date (take the long way, so as to get a little sweatier) and let your hair get all helmet-y. If you want to go all out, leave your helmet on for the entirety of the date, but refuse to discuss why you won’t take it off.
3.) My helmet keeps my hair out of my face. This is less of a benefit for people without hair.
4.) If you take a nasty spill and manage to crack your helmet, all of your family and friends who like you will take a look at it and feel compelled to tell you how great you are and how glad they are that you were wearing the helmet. (If you do not wear the helmet and wipeout like this, people will say different things, which you may or may not hear and/or understand.)
5.) A good helmet can make your shape more aerodynamic. It is no fun to be edged out in a race because your lumpy head slowed you down.