Leela, you make a reasonable point based on what CS Lewis said.
But to really makes sense of this whole issue, isn’t it necessary to first define what marriage is? What is the purpose of marriage? And in an even more basic sense…what is the purpose of having sex? And what is the purpose of gender in the first place?
Marriage is more than just a contract defining “civil union” type rights. It’s more than a document which says “A officially loves B”. It’s more than a piece of paper saying that A has the approval of society to have sex with B.
Marriage (traditional) is the most basic institution of human civilization. And until the last few years, it was always fundamentally intertwined with children. Either the actuality of children, or at least the possibility of future children. Marriage is the institution in which children come to life in a way structured by society which defines who is responsible for their welfare.
To have children, both male and female are required.
Marriage is fundamentally about procreation, and defines the restrictions on, and responsibilities of those involved towards each other and the children.
Having sex with no intention to have children, or while attempting to circumvent the natural process by which children come to be is not part of marriage. This obviously restricts marriage to male/female. This is also the reason for the Catholic Church’s stance against artificial contraception. God created 2 genders. Male and female must partner together to create more persons, in imitation of the Trinity in which the love which reflects endlessly between Father and Son becomes a new person in the Holy Spirit.
There are those who wish to engage in sexual activity outside of marriage, for the purpose of satisfying lust, or for the purpose of showing love for another person (but not to the extent of bringing new life into the world). This turns the “marital embrace” into something much less than God intended it to be. We no longer become cooperators with God in the creation of new persons, rather, we misuse our gender and those gifts to create into something merely used for personal pleasure.
I know that this issue of marriage and civil unions is a huge thing these days. I probably suggested this at one time in the past, but I’d like to recommend it again (so you can get the Church’s perspective on this). Please read “Theology of the Body for Beginners” by Christopher West. There are also similar books which might be better, but I’m familiar with that one.