tbh... tbh I got a little freaked out the two times I saw blood like that when doing the do, mostly because it's really thin and oxygenated and it mixes with more common byproducts of sex so it looks like there's a ton of it
But like idk idk I understand the blood being legitimately arousing and not just in a "oooOOOooo it's your first time!" way, and yeah this doujin was really hot