Actually, it seems that they do have same-sex marriage. Chapter 42:
“Wow,” Daphne said, sounding a little shocked. “You mean Muggles really do hate that? I thought that was just something the Death Eaters made up.”
“No,” said an older Slytherin girl Hermione didn’t recognize, “it’s true, they have to get married in secret, and if they’re ever discovered, they get burned at the stake together. [...]”
Actually, it seems that they do have same-sex marriage. Chapter 42:
By exception probat regulam it seems that there are same-sex marriages between MoR wizards.
Awesome! Good to know.