I've definitely known/heard of a lot of cats that do this... I think the porceline, marble, etc is probably cool (keep in mind, cats' natural normal body temps are above 100*F) and the shape makes for an easy curl up space.... I can't really think off the top of my head why this would be a bad problem, other than restricting the human ability to wash hands/brush teeth/use the sink at all, hahaha
but since when do our kitties care when they are in our way?! haha Fortunately, Oliver can't get into my kitchen sink (he knows he's not allowed on the counters and there's not enough run space to get his 15 pound bum up there anyhow) and the only time he hops in the bathroom sink is to lick the drips in the drain.... again, he'd prolly be too big to fit in that sink anyhow, haha
Unless you've just washed the sink with cleaning products, there's nothing wrong with it. I would find it a little odd if a cat never did, really. The kitchen sink bothers me because of the disposal, but Zissou's not allowed on the counter anyway.
I agree that it has something to do with their hot little bodies. I always like to lay on a nice stone or tile floor in the summer too. It's cooler.
My cat at my parents house lays in the sink when she gets stressed out. If I take my kittens over to my parents place, sure enough within a few minutes you can find my cat sitting in the bathroom sink!!!! I'm sure your cat is doing that for the reasons mentioned above, I just thought I'd share this perspective with you!!
Spike likes to sit in the bathroom sink, but he just barely fits. He is also obsessed with catching the dreaded bathtub drain monster (our kitchen and bathroom sinks and the tub are all connected -- when we drain water from the kitchen sink, the tub gurgles very loudly and Spike hurries off to "chase" it). Oz cannot fit in the bathroom sink, but he likes to sit on the counter and supervise from there. The cats aren't allowed on the kitchen counter, so I don't know if they'd want to sit in the sink or not -- they haven't shown any interest in it, anyway.
Next to my chest, the bathroom sink is the most popular kitty spot in the house. (Even more than the $500 kitty city!!) They love to sit in it, drink from it, dangle from it....
I have found both of my cats lounging in the bathroom tub (porcelein), but I assume it's one of the coolest places in my apartment on a hot, summer day. I have never found either curled up in the sink.