“… just wondering – why is a belly peircing ok after marrying and not before? “
Without agreeing or disagreeing with this formula, I think the reason would be as follows:
After the wedding, she can do whatever she wants, providing her husband knows about it and agrees. Before the wedding, she may not tell him, and he will never know until after the wedding (this is a private body part not exposed before the wedding). In this case, the question of mekach ta’us would arise, where the buyer feels he has been fooled and did not receive what he thought he would. In a case where the husband may say “Had I known you had this piercing I would never have married you”, this would become quite a serious issue.
Beyond that, my opinion is that the issur of chukas hagoyim as far as dress goes is quite difficult to transgress nowadays. I guess wearing a nun’s habit or priest’s collar might do it, but not much else.