Because no one ever has sex unless they are trying to conceive right?Most people understand that by nature, they refer to evolution and design. Evolutionary mating behavior condemns homosexuality as a failure of the primary goal of evolution. Design condemns homosexuality as a purposeless duplication and worse, a harmful abuse of sexual design.
A marriage between two sixty year old people is a failure because they aren't planning to have any children? What about marriages where one partner is sterile? Are they illegitimate too?
There are a myriad of reasons why people get married. Having children and marriage are not the same thing. People will have children with or without marriage. These days, it is more common for a kid to be born to unmarried parents. Marriage is about property and family rights. Period.