Second, it really isn't that hard to compare the bad effects of gay marriage to the good effects. Gay marriage is legal in 5 states now, and DC. I haven't heard of many of those "bad things" that you're worrying about happening in those states. Also, we have history to help us to determine how people may react to changes that they wouldn't necessarily like, but that really don't affect them, especially where marriage is concerned.
Also, there are some important legal benefits to being married that don't come from just living together in a married-like lifestyle. One that comes to mind, that many don't consider, is spouses cannot be compelled to testify against their spouse in court. Some others include being able to make medical decisions for the spouse, ensure that each is protected in case of separation, and makes entering into certain legal contracts easier. There are plenty more advantages to being married, but much of them come down to one contract to take the place of several.