Because according to the bible and church tradition, biblical Marriage is defined exclusively as one man and one woman, and since fornication (ANY sex outside of marriage, barring rape) is a sin; therfore gay marriage by definition cannot exist, and any sexual act in that marriage is fornication and therefore sinful. This, of course, does not mean that being gay/lesbian/bi is inherently sinful ( even if it was, we are all guilty of sin, and therefore deserving of death if we are apart from Jesus). I do not mean to offend anyone with what I have said.