In regards to "same-sex" marriage, why would a Christian quote from Leviticus 20:13?

As part of the comment thread for my blog entry The pain of my divorce, and how it shapes my position on "same-sex" marriage, and love I received the following last comments from a Christian who is using the name "Common Sense":

God's peace to you also sir.

Living in sin, no matter for how long, is still living in sin. Just because it's been for 40 years makes it OK? The word "couple" as you use it doesn't apply and I reject it.

Is it also a civil right if that "couple" is a brother and a sister, a Father and his daughter, a 50 year old and a 5 year old, a woman and her cat? Sin has, and will always be, sin. Today's so-called standards do not change the truth.

You are taking God out of marriage. There can be no true, honest debate when Our Lord is removed from marriage. You are truly scarred indeed, and I pray for you. This secularized thing you keep referring to is NOT marriage. You shouldn't mis-apply terms to make your point.

How does that feel? ...to completely ignore God and his Word when it suits you? It must make a lot of things easier.

Leviticus 20:13

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After receiving that I replied with the following:

Dear Common Sense,

There are too many ideas that come to my mind and too many feelings and emotions in my heart to post here regarding your last comments.

I have decided, in the spirit of productive dialogue, to respond to your last comments in the form of an "open letter" that I will be posting as a new blog entry.

your brother in Christ,Mario

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I ask all of you for your prayers and/or for your positive thoughts as I reflect and meditate on this issue. I shall be posting my reply (as I said, in form of an "open letter") soon.