I disagree with most of the respondents.
I have a fair number of piercings and I consider myself a pretty modest, moral person. None except my ear piercings are visable (I usually wear one-piece bathing suits, which hide the navel piercing) to anyone by my husband.
I had my navel pierced when I was 14 (not condoned by my parents) and got most of the rest (aside from the first 2 ear piercings) when I was 20-21.
My husband doesn't care one way or the other, so long as my piercings and tattoos (I also have two of those) aren't visable to others and don't interfere with my ability to get a respectable job, which they don't.
I fail to see how piercings and tattoos (most of which shouldn't be visable to others if one is dressed modestly in the first place) can be anything but morally neutral. Sure there are a lot of shady characters that have them, but like a previous respondent said, there are a lot of shady characters who dye their hair also, does that mean that hair dye is inherently immoral? I'm sure they also eat at McDonald's occasionally, does that make McDonald's immoral?
As to why one would want a piercing like that in the first place, I got it because I thought it looked cool. I liked to look at it (which I guess could be construed as vanity if you're really that serious) and I still think it looks cute. I'm sad that I'll have to take it out in a few months when my PG belly starts growing.