Okay I tried to read as much of the comments as I could until I couldn't anymore.
1) I believe FMC saw it as a mosquito bite, just because a mosquito bit you doesn't mean you are cheating on yo girl. And the vampire guy clearly sees humans as blood bag or more appropriately as a lunch. What happens in this case is very realistic as gajin just started integrating to human society.
2) This is 100% NTR bait. But look I have read Korean Roms so I can look past most sht. But this here is proof that our Mc won't get NTRed. Not only is he aware, he stopped a girl being taken advantage of for her lack of context, most importantly took action even in his least confident state of himself. He will only be a Giga Chad from now on.
3) The vampire character exists as a catalyst for our anxious Mc to take action.
**Overall great story considering they didn't make non-human characters and slapped the most cliche anime ass personalities on each of them. Every character is uniquely written depending on their background. I can't really say well written but this author is breaking grounds so he gets a pass. 9.5/10
Having just started this series I've read through some of the comments after that moment and another poster covered it well, and I think some of the comments, where doing the fan thing of being overly defensive rather than go "Yes it's good, but there's that one moment that the author fucked up but it was just that one moment" assuming it was just that one moment.
It doesn't matter how the FMC feels, what matters is how the audience feels, vampire bites are sexual, it was clearly seen as sexual in nature in the manga, it was only 3 chapters so you can't tell people they have this completely alien way of thinking, which we know won't be true. It was manufactured as you say for NTR bait.
Your comparing what started as a sweet romcom with Korean Romcoms, if I'm watching some Quentin Tarantino and the lead is driving in the car with his buddy and says he found his girlfriend getting railed by 10 other guys, I shrug, we have different expectations for different works.
It would have been a million times better if he had pushed him away before the deed with the same payoff. As another comment put it, it's the tonal whiplash, the scene seems so out of place where in just the previous chapter she was freaking out from him being close and no there's no reasonable headcanon for this to work, it was very much a forced situation by the author for NTR bait.