Loved this.
If I had a piercing fetish and had been through so many bad break ups I would fucking adore if someone kind, caring and kinda sadistic woman took me in and made me forget about all those bad break ups.
Through out the story we see the protagonist touch one specific earring again and again but she readily allowed Ao to remove that one as well. It's hyperbole for letting Ao remove all these bad unfaithful lovers from her past, and marking her in her own, much more permanent way (Though I doubt she has more than a superficial bite mark, you just can't go flaunting that shit around to strangers or friends).
If Ao was controlling and possessive in a creepy way, the one place Protag wouldn't be the day after her "claiming" her is out with her friends alone were she can potentially get support or flee.
Ao did not take protag in and lead her straight into her torture chamber. They are testing out the beginnings of a BDSM relationship. Ao herself said that Protag smiling again was her motivation. We have no reason to doubt that especially when we aren't shown anything ominous that contradicts it. She was turned on by what happened and seemed happy to be claimed in a new, unique way.
Biting happens, we have a bite tag for a reason. If you are very concerned about it I suggest you avoid it like I strictly avoid certain tags I don't like.
Bisexual women deserve our love and support too, I get that some people want to avoid seeing mlw scenes at all but not every Bisexually tagged story has that at all. Some people are acting like a woman's purity or likability is tanked if she has relationships with men. Guess what, if you're a wlw the majority of your dating pool has had relationship with men, and many continue to do. So get over it.