It is a fact that same-sex unions have been denied certain secular benefits such as joint filing of taxes. However, in their zeal to gain these benefits, they have tended to overreach.
The same-sex union community has appropriated the term “marriage” to include same-sex unions. Why could they not have been satisfied with a category of secular union that would have offered them the secular benefits they sought? That choice could have avoided, or yet avoid, the debate about the meaning of marriage.
The LGBT community seems to use terms such as “gay pride” or “rainbow coalition” with great fanfare. Labels such as “gay union” or “rainbow union” might be used categorizing same-sex unions. One can imagine other plausible labels.
Some same-sex couples have wished to impose participation in their same-sex ceremonies and related activities on others who regard same-sex unions as immoral, as in the recent photographer sued by a same-sex couple for being unwilling to provide services to them, or the pastor who refused to perform a same-sex ceremony. (Surely there is no dearth of individuals who can provide those services.)
Maintaining the time-honored definition of marriage as a union of a man and a woman will do no harm to same-sex unions. It seems reasonable to suppose that a same-sex union is of a different nature than a traditional marriage, but both unions could enjoy the same secular benefits.