Taning in the buff is natural. Remember we are not born with clothes on, society has decided that is needed. The tanning bed will have no more or less effect on the cells of your dick than they do on every other cell of your skin. While I dont advocate excessive use of tanning beds or laying out in the sun due to the radiation exposure. There is an effect in living things to radiation called hormesis, it is the odd (but handy for evolution) increase in cellular health with small doses of radiation. So as with everything in life, moderation is the key. Tan with what ever you want on your body, just remember tanning yourself into leather is not a good idea.
If you are feeling paranoid about it I recomend you visit a radiologic supply company and purchace a gonad shield for x-rays and use it in the bed.