Just a simple outline for you:
- First off, even if homosexuality is genetic, does this then make it acceptable to engage in homosexual acts? No. Anger or perhaps the predisposition toward anger might be genetic, for example, but it doesn't make it right to murder someone.
- Some people have argued that there is a significant difference in the size of a particular region of a homosexual's brain relative to a heterosexual's brain. Even if this is true, how do we know that this is the cause of homosexuality rather than its result?
- Even if homosexuality is genetic, why should homosexuality be considered permissible on these grounds? Or even considered "normal"? There are many diseases such as certain forms of cancer which are genetic. But these diseases are not therefore "normal." In fact, that's why they are "diseases," because they are not normal; they are not markers of health.
- Even if homosexuality is genetic, it's not as if everyone with a gay gene is necessarily controlled by it. Even if there is a gay gene, it doesn't then follow that one is therefore compelled into homosexuality.
- At best, the existence of a gay gene might mitigate sin in some sense, but it would never excuse let alone condone sin.
- Now, if homosexuality is genetic, and if this in some way excuses homosexuality, then wouldn't the "inheritance" of our sinful nature from Adam excuse all sin -- which would be patently false?
- We've been talking about genetic factors (nature), but let's not forget about environmental factors (nurture). If we were raised in a home with physically and emotionally abusive parents, would this excuse our being physically and emotionally abusive towards our own children? No.
Male-on-male and female-on-female sex is blatantly against how we were physically created. It overturns our sexuality. As John MacArthur once asked, with a touch of humor, "Do I need to draw you a picture of how a man and a man or a woman and a woman cannot possibly physically 'fit' together?"
Homosexuality cuts against God's purpose for humanity as "male and female," as part of a family, as part of society. Men and women were created with distinctive roles. This is not a sexist statement; this is how God created us. This recognizes who we are as males and females, as husbands and wives, etc. And the family is the most fundamental building block of society. Note, for instance, that Eve was given her name because she was "the mother of all living" (
Gen. 3:20). Her name is tied to her role as mother. Homosexuality does away with our God-given roles as men and women and as families.
There's also the related danger of homosexual couples adopting children. Imagine the possible affect on little Johnny or Jane growing up with two dads but no mom or two moms but no dad, to say nothing of the affect on others. I'm not saying it's a foregone conclusion but from what I've read and seen there does seem to be a high degree of deleterious effects in exposing and raising children within the context of a homosexual lifestyle.
According to the Bible, there's a sense in which homosexuality demonstrates the height of sinful, rebellious man (à la Romans 1).