Do homosexuals need to be married?
It would easily be possible to survive without being allowed to marry, if that is what you mean. But it would not be fair from a legal standpoint, just as it was not fair to deny women the right to vote. The fact that people could get by without it does not mean the situation should go on unchanged.
It is true that in humans and most complex animals, a male and a female are required to reproduce. By the Catholics’ religious definition of marriage, gay marriage is invalidated by the relative infertility of the couple. So far as I know, few are arguing this point. As someone who argues for same-sex marriage, I concede this point.
But the government of the US is not a religious institution. Other religions have differing definitions of marriage: some say that infertility or direct prevention of children is acceptable, others do not. Some say that divorce is possible, but others forbid it. Because the government does not have an official religion, it cannot choose one religion’s definition over another. The US government was created as an administrative authority only, and does not have the right or the authority to decide matters of sexual morality.
As an administrative authority, the government has a legal institution which, like the Catholic sacrament, is called “marriage.” This is a legal status which has been created to assist individuals who want to live together for long periods of time in doing so. It makes some things easier, such as co-ownership of property, visiting rights, and the legal requirements of the married towards children concieved by the two or by only one of the two. Because it is an administrative status only, there is no need for the non-religious authority in charge of it to concern itself with the fertility of the couple, their attitude towards raising children, or their present attitude towards ending their union. So, the government has no administrative reason to deny this status to same-sex couples.
So we gays could live without this status, sure. And if for whatever reason a straight couple was denied civil marriage, the same would apply to them. But Uncle Sam has no reason to make life harder on any one group by denying them this status.
In short, while the original argument shows why Catholic marriage is impossible for gay couples, it says nothing about whether a non-religious institution should allow gays the civil status of marriage.