It’s about the breakdown in traditional gender roles. In a “traditional” marriage the man goes to work and the woman does household stuff.

Put another way a traditional western household needs both a man and a woman, and it actively doesn’t work if you don’t have both because a woman would not have been able to go to work and make any sort of reasonable wage capable of taking care of a household and a man would not have been able to respectably take care of a household. Victorians would not have considered two men or two women living with each other a household or a marriage because part of their definition of a marriage, men, and women are their social roles.

Gay/lesbian marriage only makes sense in a world where some semblance of gender equality exists and these kinds of social roles aren’t so gender exclusive. The original comic is fearmongering the possibility of a more equal social order where it’s even possible to consider women being able to become enough like men that they could marry other women.