I'm addressing this question to Christians who believe that God is a utilitarian, that is, that God has designed His creation and attempts to guide His creatures with the aim of maximizing some divine utility function. As with the case of the laws of Physics, the full details and intricacies of God's utility function are probably unknown to us, but we can sort of intuit from experience that wellbeing and suffering are likely to play a crucial role in God's evaluation of utility. I already entertained this idea in a previous question, where some of the answers confirmed that this is the view of at least some Christians.
So, if we assume that God is a utilitarian, why is homosexuality suboptimal? Why would the existence of responsible homosexual monogamous marriages detract points from God's utility function? Why is the case in which only heterosexual couples exist optimal?
Note that we can extend this question to more convoluted alternatives, such as bisexuality or bisexual polyamory/polygamy, where biological reproduction is still viable because some of the relations are still heterosexual in nature, while others are not.
Regarding the question of whether God is a utilitarian, I found a relevant publication on this:
Is God a Utilitarian?
Published online by Cambridge University Press: 24 October 2008
In a well-known passage Mill defends his utilitarian theory against the objection that it is a godless doctrine. He argues,
If it be a true belief that God desires, above all things, the happiness of his creatures, and that this was his purpose in their creation, utility is not only not a godless doctrine, but more profoundly religious than any other. If it be meant that utilitarianism does not recognize the revealed will of God as the supreme law of morals, I answer that a utilitarian who believes in the perfect goodness and wisdom of God necessarily believes that whatever God has thought fit to reveal on the subject of morals must fulfil the requirements of utility in a supreme degree.