Why should I agree with gay marriage?no_redirect=1

Disagree. Your teacher is correct. The same arguments for homosexual marriage "people who love each other should be allowed to marry" applies to incestuous marriage, pedophilia marriage, and polygamy.

It is not a rights issue either. Since this has come up several times today, I will cut and paste an answer I put in a previous question here.

Currently homosexuals have the same rights as heterosexuals when it comes to marriage. The laws on marriage do not say you can marry whoever you love nor do they say you can marry whoever you want. They allow a man, whether he is homosexual or heterosexual, to marry a woman, whether she is homosexual or heterosexual. Why get married? For economic reasons or social reasons or whatever. Sure, it is nice to marry someone you love, but for a majority of human civilization, arranged marriages worked just fine and they did not involve love.

So you can see it is not a "civil rights" issue. You already have the same rights.