Taphel's manga / #Adorable(43)

Ummmm I have but one question. If it doesn't matter if the royals marry a girl or boy. How are they meant to get heirs... you know... carry on the royal lineage? One of the most important reasons for marriage was to produce an heir. If this royal line is full of same-sex marriage as well, then how has it continued? Do they just take detours through the King/Queen's siblings, or do they just drag up a random man/woman for the ruler to bed to get a baby? (obviously not random, but you get what I mean..) How on earth has this royal bloodline survived?! In a normal same-sex marriage I suppose you could adopt, however that doesn't strike me as something that would be seen as acceptable in a royal marriage, with the fate of the country and the support of the public in consideration - look at how annoyed some were that the 'Queen' was a common Japanese (I'll admit that marriage between royals and common public has forged greater bonds though. Look at Kate and William's wedding). I just don't understand how they expect this bloodline to survive. I just don't know. If you can, will someone please explain to me why this seemed like a good idea? (I have no problem with the manga, I quite enjoyed it. I'm just not happy with the logic behind it.)