I think I made a post about this a while back but I’ll say it again since it’s been a while. Short answer – it’s a fantasy AU so I just kind of figured I could do what I want. Long answer – it’s a fantasy AU and humans evolved differently because something something magic. Basically, pretty much anybody can get pregnant or impregnate others regardless of gender. The details of how this works depends on what kind of level of ‘unconventional biology’ you are comfortable with, but I find the whole process a little bit squicky myself so you probably won’t be seeing any in depth pregnancy headcanons on here. Since reproduction is basically an ‘anything goes’ kind of deal, most people in this AU are bi since I feel like that would make the most sense. Gender roles (fashion, parenting, occupations, etc) also tend to be very flexible, even historically. There’s probably more I can go in depth with, but that’s the basics of it. 

The actual answer is because I like designing fankids who have traits from both parents and wanted to be able to do that even for same sex couples, so I made up an explanation that fit into my already established worldbuilding for this AU.