Replying to Same sex marriage question

i'm doing a short debate paper on gay marriage. I'm not really for or against same sex marriage, i just don't fully understand peoples problem with it. it seems like most gay couples just want the legal documents saying that they are married so that they can get tax deductions and have other benefits. it doesn't really seem like gay couples really care about the traditional side of getting married in a church. all these religious people don't want gays to marry cause the bible says married should be between a man and a woman. so why not just let gays marry for legal reasons and just not let them get married in a church?