One society thinks multiple wives is ok, another it isn't. One thinks wedding teenagers and preteens is ok, another doesn't. In the US until recently puberty was an indication of when one was ready for childbirth.
Are we more moral now by postponing adulthood?
I think there's a difference between these types of issues and asking "does it harm people? does it sacrifice greater good for beings for my own desires?"
I personally think the problem is most people seem to get hung up on thinking of cultural norms as morals, and then forgetting morals and ethics all together.
Hence, we get people who are honed in on the gay marriage issue in the US and meanwhile we have a large percentage of kids in poverty, lousy health care, and high violence rates.
As for the puberty and childbirth issue, that is easy... In most societies and times, puberty for women was around 15-18 years old. Not that bad of an age for marriage, if the culture prepares people to hit adulthood at that time. In our contemporary America, due to the wonders of high-fat foods and hormone-laden products, our girls are hitting puberty at 9-10 years old. Um, there's a problem. Because there's no way to make a 10-year-old capable of adult judgement- their brains aren't finished with construction yet. Not to mention 10 year olds typically can't survive childbirth without a lot of assistance.
One can be a relativist on a lot of issues and still uphold universal ethics. Many folks are culturally relativist on issues that harm no one and promote universal human rights on issues that would harm others. I.e., I don't support slavery, and I don't care if it's the "norm" somewhere else. Just because something is a norm doesn't make it ethical. However, I don't care about polygamy, so long as all parties are consenting adults and women aren't oppressed. Doesn't work for me, but can't see how it harms those who agree to it.