Man made the definition of marriage, we can change it if we want.The principle of marriage has always been between man and woman - that is within nature that spanned for more than millions of years. Marriage isn't just about love; it's also about reproducing so that more generation is born. Homosexuality lacks the latter. Yes, I'm opposed to it. It changes the principle of life, it is a sin, and it opposes laws of nature.
So infertile people can't be married? Homosexual people can also still reproduce. Donating sperm, eggs, surrogacy, for example.
Against the laws of nature? Sexuality isn't manmade, it's determined completely naturally.