Two thoughts:
1. Many gay men like to show off at the beach, get tans, etc. More exposure = higher risk.
2. Gay men generally pay a lot of attention to their physical appearance and thus are more likely to notice/go to a doctor over something odd looking on the skin than straight men.
Need more detail.
Just what areas of their skin are getting the cancer? Asking for scientific data.
Fingers, toes, scalp, back, thighs, . . . , better stop here.
BINGO
BANGO
HOMO!