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Any argument based solely on reason can be argued against. This creates a couple of problems:
The real reason I as a Christian oppose h*m*sexual actions, including h*m*sexual marriage, is because I believe God has warned us that it is sin – and therefore will lead to unhappiness for individuals and for society as a whole. Other reasons are tangential and can be argued against, but faith can't be argued against very easily. Why would I leave the real reason for my opposition out of an argument?
In any debate, we are most convincing when we speak from the heart. Whenever someone speaks truth from their heart, it gives the Spirit an opportunity to testify to anyone willing to listen. And if a person isn't willing to listen, no argument will convince them.
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Because Protestants are fideists.
Fr. Hardon, SJ, defines "fideism" in his Catholic dictionary as:
A term applied to various theories that claim that faith is the only or ultimate source of all knowledge of God and spiritual things. The name was originally coined by followers of Kant (1724-1804) and Schleiermacher (1768 -1834), both of whom denied the capacity of reason to know God or the moral law with certainty. (Etym. Latin fides, belief; habit of faith; object of faith.)
Protestants believe in sola scriptura; according to them, their private interpretation of Holy Scriptures trumps philosophical arguments based solely on reason.
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"Protestant" means a believer in sola fide and sola Scriptura. As such, not only is it possible for a Protestant to use natural law arguments, many have. Although most of the reformers were Divine Command theorists (Luther and Calvin, although Calvin is weird), not all were (natural law was influential among both the Catholic and Protestant American Founders, although with a clear Lockean (Divine Command) influence as well). Many Protestants today are natural law theorists, although not all are.
For information on the Founding Fathers: http://scholarship.law.nd.edu/cgi/viewcontent.cgi?filename=5&article=1001&context=naturallaw_proceedings&type=additional
For an article on Protestants and Catholics regarding Natural Law: http://www.firstthings.com/article/1992/01/002-protestants-and-natural-law
Christi pax.