Something-something, insects have pheromones, something-something ,Sparda was vaguely insectoid (that's enough for me), something-something, Dante and Vergil are Sparda's sons.
Or in other words Dante and Vergil are able to produce pheromones and because of their half-human nature those pheromones affect humans too. In small doses it creates or heightens attraction but in bigger doses since it isn't meant for humans it could become harmful, cause arrhythmia, usually tachycardia, internal bleeding due to burst vessels, nosebleeds, nausea.
Dante gives his best to keep it under control yet while he was younger he has accidentally used them a few times making people chase him around and causing a nosebleed or two. Also probably isn't too keen on the idea of taking choice from people like that, since the pheromones simply erase their will.
Vergil, since he is more in touch with his demonic side, is very aware of how those pheromones work. Might've even used it here and there to get what he wants when Yamato would've been too exposing. He isn't really concerned with the moral conflict here, if it works it works.
And Vergil truly doesn't understand why Dante is so horrified when he sees him use it on someone. Surely, drawing his blade wouldn't have been better. Right?
