Indeed, there is no lawful reason for denying same-sex marriage. I don't see a world where a bonafide Objectivist could argue against homosexual consensual matrimony.
Just to note a few similarities of tantamount importance:
"For a woman qua woman, the essence of feminity is hero-worship--the desire to look up to man." - Ayn Rand
"If you have a more feminine sexual essence, your professional
life may be incredibly successful, but your core won't be fulfilled
unless love is flowing fully in your family or intimate life." - D.D.
Both authors share the same ideal of masculinity and its opposite.
Another quote by D.D:
The imagery of Dominique is perfect here, and Rand's characterization of how Roark responds to her is just so consistent. That in Dominique's most brutal "testing" her secret desire was that Roark could not be broken...the harder she pushes for hurting Roark, the more she admits to herself of the depth of her love for him, that even when she succeeded she still could not hurt him and that he still completely dominated her in mind and body.
And the later part of the book when Dominique leaves Roark and returns to test him and see if he will abandon his purpose, in that moment truly acting as if she wanted just that, and Roark of course knowing that deep down it would be a betrayal of his own values, and that Dominique would grow to resent and hate him for it if he would submit to her impulse...