"What about people who love their pets?"?!
Well then...
There's a difference between a person marrying a person and a [erspm "marrying" an animal, because there is a legal difference between a person and an animal. There is no legal difference between people of different races. There is no legal difference between people of different sexes. There <I>should</I> be no difference between a person marrying a man, or a person marrying a woman.
If you can explain to me why the 14th Amendment covers interracial marriage but <I>not</I> same-sex marriage, then I'll understand why you think an act of Congress would be required to legalize it.