Because men see it as having their nuts completely cut off, in short order, castration like we do with our domesticated pets. It's not a pretty picture to paint in your mind. I just wear a condom, it's that simple, plus it protects you from most of the sexually transmitted diseases that may occur.
I don't need to go raw dogging any woman, regardless of what she tells me and I don't buy the bullshit about how it "feels".
I'm a responsible adult, always have been and always will be. Do you honestly think some irresponsible person is going to take precautions? Look how fucked up it was with a simple vaccine that prevents you from dying. The 'mom's for liberty' set would be up in arms if little Johnny had to endure some type of procedure.