I don't see any reason why marriage can't be between two adults who love each other, regardless of gender. Some of the arguments I have heard against gay marriage are religious, some not. Here's some of the ones I've heard:

Marriage is for the purpose of procreations, since same-gender couples can't procreate, they can't marry. (So what about those who can't have children for whatever reason? Does this mean I shouldn't get married because I'm past childbearing age?)

If we allow same gender couples to marry, then we will have to allow people to have more than one spouse, to marry children, animals, etc.

The Bible forbids it - most of the arguments come from all those lists of laws in the Penttateuch. Why pick just some of those laws to keep and not the others?