Modestinus definition of marriage

"Marriage is the union of male and female and the sharing of life together, involving both divine and human law." - Modestinus said.
On that phrase straight marriages are based. I was told many times about that at the university.
Why should they act so aggressively to the gay/lesbian marriages?
Though, I do not have my answer for this because of my brains filled with all the humanity's prejudice.
1st. I think that gay/lesbian marriages are normal, I do not think that such a prohibit has any grounds. Why should anyone say anything if people love each other and want to be together officially? It will not hurt anyone.
2nd. If they really want to divorce that will suck but mah a lot of straight couples divorce.
It is only in our minds that we consider something to be better than other thing. Believe me, gay and lesbians are normal people, like straight. That is definitely not the reason to consider someone to be abnormal. We, as humanity, have other problems other than the hatred to the people near us.