Actually from what I hear, in japan it's normal if they married their cousin / first cousin(?), so maybe because of different culture. But this is just manga, we're already at the age that can differentiate between fictional/non fictional/just manga. So just enjoy it (=・ω・=)
Unfortunately a lot people aren't like that hahaha
I thought it would be cute from the start, because he was complimented on being cute and all. But it turns out uke is too accepting, and seme is too selfish. Although the event are very realistic, but since it's a manga I won't bother too much about it.