Gay marriage was, and still is, illegal around the world for centuries, especially after the spread and growth of Catholicism and then Christianity.
Marriage holds a massive spiritual and cultural significance for people around the world, no matter their religious background. Marriage is a goal for many people before they even understand their own sexuality. For generations, people have either had to give up or compromise on a years' long dream in order to conform to heteronormative society. If both parties are consenting adults, there should be no reason someone should not have the right to share a union with someone they love with legal recognition and all benefits provided by federal law. Many national governments around the world have recognized this fundamental truth over the last few decades, overturning old laws and writing in new ones that allow people to marry the consenting party of their choosing. A person who marries the person of their choosing is more likely to have a happy, successful relationship, whether they are gay or not. So, there is no valid reason why a government separated from religion should bar same-sex couples from marriage.
There must be limits on marital choices. A person may not, for example, marry their relative. To protect the institution of marriage there needs to be clear restrictions on what can and cannot be permitted under the banner of marriage.
[P1] People have a fundamental right to marry who they love. [P2] There is no reason this should restricted for same-sex couples.
Rejecting the premises
[Rejecting P1] Marriage is not a fundamental right. [Rejecting P2] Marriage is not a right without restriction.