There is a difference between somebody getting a body piercing and getting married. Marriage always involves more than one person and it also involves society.
Yes, it involves more than one person. And, in this case, both parties want to be married.
As for involving society, I would argue against that. Since the couple in question is probably living together already, changing from living together to married would only effect them. They would receive the benefits the gov't give married couples. But society would be uneffected.