Natural law is not purely religious, no, but when you use it as a defense to forbid two men to marry, it becomes one.
(1) Natural law is not the only argument against gay marriage, just one component of several component arguments.
(2) Natural law is actually woven into our secular law. Has been true historically; continues to be true today.
(3) Even the most “secular” law is built from a combination of reason, logic, and human experience. Since the institution of democracy, law is not arbitrary or monarchical. However, people tend to forget that many laws even during reigns of medieval monarchs, also incorporated these essential elements of logic, reason, and human experience, within a framework of justice. Note, however, that justice must be seen as comprehensive for all parties involved, and for society as a whole. That latter fact is the basis for the following argument, which i.m.o., is the strongest argument against gay marriage…
(4) The common good of society. This has already been addressed by others. Gay marriage does not promote the common good. It promotes an individual good or desire (as opposed to individual
rights). The notion of individual justice, individual rights, can be and (i.m.o.)should be resolved by granting equal privileges to gay couples (that heterosexually married couples have) without the title of marriage, when two people of the same gender seek to associate with each other.
I think that same-sex couples should have inheritance rights, hospital visitation rights, tax deduction rights, insurance rights, bereavement rights, etc.
However, the way our society is structured, the term marriage always implies the privilege of parenthood. And gay parents do not promote the common good of society. This has been discussed on other threads, so I’d rather not hijack this thread to discuss gay parents.
Secondarily, the term marriage has religious connotations and implications, regardless of its civil origin. It has evolved to signify religious approval, religious status.
If civilly-united gay couples wish to have their union blessed and given additional religious status, let them seek out such religions which will provide that, and quit demanding that the Catholic Church change to accommodate them.