^Nah, you can get married to way more than just them. Basically everyone opposite gender is romanceable (ie playing male Byleth you can get married to every female characters depending on the route). For gay options, female Byleth can get married to Edelgard, Rhea, Dorothea, Mercedes, and Sothis. Male Byleth can get married to Linhardt, Jeritza and Yuri.