Apparently, Yuuto is the secret son of the Ijuin family’s deceased vampire king. Because he’s technically the new vampire king, Yuuto is free to take any vampire woman he desires as his mate (likely a nod to the stories about vampires taking multiple wives as they saw fit). If I have my facts straight, because Marie is the eldest daughter of the Ijuin household, Japanese tradition (which governs literally every fucking social norm in Hentai series like this) dictates that she would be the one to be married to the first eligible suitor to become available (in this case, Yuuto). However, I do agree with you. Serena claimed first dibs on Yuuto. Marie needed to back the fuck off.

On the other hand, Yuuto is getting a hot threesome with two uber-busty vampiresses who he plans to marry. So… doesn’t matter, had sex. XD