I'm not Christian. Many people are not Christian. Not all Christians agree that a marriage has to be a man and a woman.

Christianity does not have a monopoly on marriage.

Christianity has only existed for...not even 2000 years. Biblical scholars now think Jesus was born somewhere between 6 and 4 BC, i.e. a few years earlier than other guesses. He didn't start teaching until he was 30 which, I will note, is the minimum age to be a rabbi. Yes, he was a rabbi. So, Christianity is not over 2000 years old, but that's me being a pedant.

2,000 years is nothing. Cleopatra lived closer to us than to the construction of the pyramids.

Modern Homo sapiens has existed for about 160,000 years, and archaic Homo sapiens for 140,000 before that...300,000 years old,

Christianity is a blip.

Christianity does not have a monopoly on marriage.

I will turn this around.

It's not not a marriage because you or a religion to which the people in the marriage don't belong say it isn't.

Current estimates for worldwide religious adherents in 2020 per wikipedia (which is a decent source for this kind of thing), by percentage:

Christianity: 31.11%

Islam: 24.9%.

Secular: 15.58%

HInduism: 15.16%

Buddhism: 5.06%

Chinese traditional: 5%

Other ethnic religions: 3%

African traditional: 1.2%

Then there are a ton of others at under 1% each, such as Sikhism, Judaism, Shinto, etc. Christianity has the most adherents, but the majority of people on this planet are not Christian and don't care about a Christian definition of marriage.

You don't get to force your religion's "values" which aren't even agreed on by every practitioner of your religion on the entire planet.