Now explain why a sterile man and woman or a pair of hetero 65 year olds marrying isn’t “against nature” or a thwarting of the primary “reproductive purpose” of marriage.
Biblically, sex should only take place within a marriage. If it takes place within a marriage, the sex is sanctified, and non-sinful. Any sex that takes place outside the context of a marriage, is inherently sinful. That means adultery, abuse, and so on. Same-sex individuals simply cannot marry within the Biblical meaning of the term, which means that their sexual activity will not be sanctified, and will therefore be a sinful action by nature of what it is.
Being a homosexual or feeling attraction to your own sex is not sinful - it's a very difficult temptation that, with God's help, you are supposed not to give into. It's acting on this same-sex attraction that is sinful, in much the same way as acting on attraction to your neigbor's wife would be sinful.
That one’s always fun to watch.