Perhaps it would help to illuminate the issue if we asked ourselves "what is marriage", in other words
why do we have it and what is the goal or benefit from it?
Is it just so a heterosexual man and woman can have a 'special status' in relation to homosexuals and single people? If this is the answer then I see no reason we need to continue the practice at all.
Is it a ceremony of commitment between people in an effort to show devotion to each other for the rest of their lives? If this is the answer then I see no reason that any group of two or more consenting adults shouldn't be allowed to commit themselves to the well-being of each other and therefore legally share in the right to the participate in the ceremony.
Is it just for monetary purposes such as legally acknowledging your offspring and making them the heirs to your estate and such? If this is the answer then marriage can either be eliminated and replaced will other legal documents such as wills, or it should be open to homosexuals too as they have the same monetary concerns as heterosexuals do.
My guess would be that it originally existed to promote permanent, lasting family units for the
purpose of having a stable environment for the raising of children. Can anyone think of a better reason it exists? Is there some other reason I may be missing?
If marriage is all about helping to raise kids in a steady and secure environment then why should it just be limited to heterosexuals? Homosexuals are allowed to raise children and as long as they continue to be allowed to do so then why would we want to deprive them of a tool that may help encourage stability for their children?
If it is about helping kids, then why do we allow couples without children to participate in the institution? Many couples don't want kids, are physically unable, are too old, etc. Why should they get special status and privilege when they have no children to benefit from the marriage? Wouldn't it then make sense to only allow marriage to couples that have children on the way, are in the process of adoption, etc. and if their attempt to have children is unsuccessful then the marriage can be annulled as the childless couple is no longer in need of special treatment on behalf of children they don't have.
Why don't we
define the goal of marriage and then grant it to those who are seeking that goal, without any interference based on our own personal religious or sexual-orientation biases hindering logical thinking.