I see both sides of the argument on this one. I myself have 1 tattoo and am planing at least 2 more, likely 3. I use my tattoos as a form of religious expression, but I also see how they can get outdated. I have a spider on my foot I got in first year university because I thought it would be a good idea. Now I'm thinking of getting a second one to point out how foolish the first one was! (the idea is a snake on my ankle, about to eat the spider, and "Everything Changes, but Nothing's Truly Lost" written in it).
While yes removal does exist, it's expensive and can leave scars. So it's there, but I think only in extreme circumstance. I think tattoos can be beautiful, but I've also seen a lot of trashy ones. And you do grow old! Those roses on your boobs might become hanging baskets by the time your 40 or 50!
I'm planing mine carefully and dating them throughout my life. My biggest is going to be when I'm at least 40. So no, I don't think they're a sin, but I do think you have to try and be responsible. A tattoo of hot pink brass knuckles on your shoulder isn't going to be as great down the road... hell, maybe even that cross will get outdated?
Piercing are another story. I like piercing because you can take them out, but beware of scaring!