Every argument used for same sex marriage applies to every other type of marriage.

Gays are arguing for the right to marry the person they love. They can already marry an opposite sex person they do not love. Is there a right to marry the object of your affection? Straights do not have that right. Here are the problems with that argument:
1 the object of your affections may not have the same feelings.
2 the object of your affections might already be married
3 the object of your affections might be a close relative.
4 the object of your affections might be a minor.
5 you may be already married to someone else.
6 the object of your affections may not be human.
7 the object of your affection my be an inanimate object.

Need I go on? This is a guise of equal rights.