No, it’s not sinful. I had one, and have many piercings now, though my navel piercing didn’t work out. I still have, and will always have the scar (it migrated outwards and so left a bit bigger scar than one that would have healed normally). Navel piercings are temperamental. Even after they heal, they can become unhappy if bumped/kicked/etc. A lot of the time they migrate/are rejected. I don’t necessarily regret doing it, I just don’t think it’s very practical.
It is possible to keep the piercing open during pregnancy – most people either take it out and see if it closes up, or put a flexible bar in.
As for ornamentation vs. not letting anyone see, piercings don’t have to be a public thing. There can definitely be a “point” to getting one, even if you don’t intend to show it off.
Bottom line: I think other piercings are more practical/less hassle. Try to think of why you really want it. If the purpose can be achieved by something other than a piercing, that may be a better route. Just make sure you’re aware it might not work out, as navel piercings are notorious for getting rejected.
