I know many lads these days tan their bums at tanning salons. I prefer, however, when their backsides are paler than the rest of them and a visible tan line can be seen.
I know I'm probably in the minority with this opinion, but for me it emphasizes their nakedness. There's something inherently erotic about it for me. I suspect this dates back to my teen years, when naked male backsides always had visible tan lines. For me, a man isn't truly naked until you can see that pale swath.