Honestly, I'll still continue reading to see where the story goes but that panel is pretty obviously NTR-baity, especially after finding out that it originally didn't even have that text saying it's just in his imagination.
Arguments arguing that people seeing it that way are just disgusting, creepy incel nerds just strike me as either holier-than-thou posturing or just being upset about people being negative about a series they like.
At the end of the day, this is a manga written by a human who is including a romance element in their story and perfectly understands how humans reading it would see that scene as intimate (and it's literally drawn to look intimate.) Also, as manga readers (or just anyone acquainted with vampires in fiction in general), we should be well acquainted with how vampires drinking blood from the neck is portrayed with sexualized undertones (and sometimes even overtly.)
And being put off by a heroine being portrayed in an intimate moment with someone other than the love interest isn't just being upset that "their waifu isn't pure." Even in romance stories where the female love interests aren't a virgin, being intimate with someone else while there is development of a relationship is seen as skeezy. Like even if you're OK your partner had other partners, a normal person would only be ok with it if that's all just in the past.
