Probably because uke was even more nonchalant receiving his harassment than seme was when dealing it. Even I think uke’s response of biting back, casually following seme to his home, and just not actually fighting at all is interesting. I’ve seen so many “normal” responses such as flinching, inward screaming if not just being loud about it, terrified monologues of how much the other person is a freak and how they need to get away ASAP. On the other hand, uke here was also instantly, albeit subconsciously attracted to seme even while finding him weird. And maybe it was also the first time for seme to find someone he wanted to bite and who wanted to bite him back.
More than wanting someone to bite him back, he didn't want to hurt that person. Even tho every fiber of his being is telling him to do so. Many have left him because of this and he was okay with it. He just moved on to the next one. With Yuito, he really wanted to treasure him and not let him go.
Yuito in return feels this and reassures Mitsu by biting him as well. To somewhat ease his conscience.
I don't understand from the start why the seme suddenly was attracted to the uke. I get that it's abt his appearance but it still doesn't make sense why he's possessive from the start.