I think it's got to do with the fact that penises are in general, pretty ugly bits of flesh.
Speaking for myself, I'd rather see a nice smile and a set of beautiful abs than a penis any day. I know I'm not going to see that many beautiful abs around here because you're all just ordinary everyday people like me who don't have chiselled bodies. There's been some awesome smiles from the fellas though.
Anyway, I'm off to look at porn.