Skin cancer is a great reason. Just lost my lifelong friend at age 38 to skin cancer he had a 3 month old baby and was diagnosed stage 2 a year earlier.
I went to the beach and wore sunblock (spf 30) all day, but I still burned. I was surprised how sensitive my skin was. I'm pretty light skinned but before my pregnancy, i usually only put on spf 15 and would be fine. I even burned on the tops of my toes, it was horrible!
You can get a nasty sunburn when you're pregnant from tanning.
I went fishing for a few hours one day and since I never wear sunblock, I didn't and sure enough, I was the only one that got a burn, no one even tanned but me, it sucked. lol
Because a lot of pregnant women are more sensitive to the sun and burn very easily and I know I also get fevers when I burn. I wish I tanned instead :)