There are a couple of flaws in your reasoning that may require you to re-think your position. Just because marriage is defined a certain way doesn’t mean the definition can’t be expanded. A ‘homemaker’ used to mean a woman but now means men as well. Marriage does not naturally produce offspring. There are plenty of married couples who can’t have or don’t want children. Should they still be allowed to get married? The fact that anectdotal evidence shows gay unions are not monogamous is troubling, however it’s possible that because no formal marriage is possible for them, it leads to the degeneration of relationships. Therefore, marriage would be a desirable goal for everyone.