This story bases on Chinese tradition which they drink blood of another to swear. Uke wanna make a blood-vow to ensure Shinobu that he'll love him ever after too.
http://www.mangago.me/read-manga/meikon/mrs/manga/meikon/v-1/4/37/
http://www.mangago.me/read-manga/meikon/mrs/manga/meikon/v-1/4/38/
Your question is not annoying but the way you badmouthed back to someone nicely apologized is very very annoying. ( ̄へ ̄)
Many people made this kind of mistake but only few were brave enough to apologize so we should appreciate their nice manners.
This is a really stupid question, but it's bothering me..... Why did he want Shinobu to bite him?