They cannot fundamentally, though.
If marriage is for procreation, they are not fit to marry. That is what it comes down to.
There are several instances in Scripture that point to why we support marriage with the need for a man and woman that, in principle, can have the ability to have children even if fertility should make it impossible.
The first instance was of Abraham and Sarah: a man and woman who were obviously past childbearing age. I suspect you don’t believe in the supernatural, but the text notes a supernatural event occurred which suspended the natural infertility of Sarah yet
still required the natural genetics and marital embrace of sex. Sarah had a child.
The story of John the Baptist is also similar. Christ’s birth is a special case but it still followed the need for genetics to work–
male RNA from a supernatural source combined with female RNA from Mary.
Homosexual unions cannot naturally make children for they aren’t complementary in genetics. Why is logical for the state to sanction a union that is more costly in bureaucratic requirements? Just for one “family,” you need two same-sex people to be legally recognized, at least one other adult to add genetics for a child (surrogate mother or sperm donor) OR a male and female couple that conceived and brought a child for adoption by the same-sex couple, AND at least 1 judge and 1 doctor to make much of this happen.
And through all of this, the rights of the child in this relationship are tossed out the window because the child is treated as an object legally owned by two people with no more than 1/2 genetic heritage, if any. Not a single human on this planet was born from a same-sex family through natural
in utero means. None. Why must this change? To satisfy the desires of someone?
History is replete with wars and other conflict, all because someone wanted something they had no right to have nor should have tried to obtain through coercion. The same-sex ideology is not the Civil Rights Movement of the 1960s, which emphasized the natural place of African Americans in the genetic and social community. The gay community has achieved the important need to not be bullied or persecuted simply due to their sexual activity. But they cannot have children at any time in their lives together, and that is what marriage is about from a state-sanctioned point of view. From a Christian point of view, we add that children grow best with a mother and father in place to complete this.
The gay community is bulling others to recognize a scientific impossibility just because they want it and require others to see their “truth” or face legal action. That’s pretty childish.