Why should I be taxed differently because my significant other and I decided to sign a piece of paper?
Very valid questions. I am not sure that I can answer them but perhaps I can provide some perspective:
It is in society's interest to promote the creation of new members of society. The majority of children are created through a male and a female having sexual intercourse.
Society has an interest in the child being reared in a secure environment, hopefully one that is positive and nurturing so that the child becomes a healthy and socially well-adjusted adult.
One of the methods accepted by society at large for encouraging the creation of children and providing for their well-being while becoming adults is to formalize what a "family" means in its most basic sense. Marriage is a tool for "encouraging" the maintenance of this familial construct.
It is not surprising that people are resistant to swapping out portions of this construct for others, such as artificial insemination, gay marriage, adoption, etc. Some swaps have some very persuasive arguments for why they negate the entire concept of marriage as a tool for family creation and social stability (gay marriage), others do not, (artificial insemination, adoption).
For myself, I am generally for what is known as traditional marriage. I do not really care what gay or childless couples do (not that they are equivalent) or what benefits they can accrue by "aping" traditional marriage. All of these people who are yelling and screaming about it all just seem absurd to me. Keep your eyes on your own business and stop being jealous or envious that someone else might have it "as good as you".