Since Rider and Miria are a married couple and both Moris mom... Does that mean they're a ...

Dude rider is a man

love my lesbian couple (≧∀≦)

Nope, rider is genderless, they're from a species where genders don't exist and rider themselves doesn't understand the concept of genders nor the concept of male and female since genders don't exist in their species and Mori is the first person from a species where genders do exist that rider ever met. Rider genuinely is a genderless person from a species that reproduces asexually who didn't even know genders are something that exists until the finale of the this manhwa. The only gendered aspect about rider is that they want to be their adoptive child's mother instead of their adoptive child's dad. That's it.