Maybe I don't understand women's perspective, or maybe such a thing doesn't even exist and it would be up to the individual, but from where I can see, Rai was acting like a total scumbag. He was basically boasting in front of the whole class how he had done this and that with a meek girl like Riku, who, as far as the classmates know, isn't going out with anyone. All that only to piss off another guy. Rai didn't give a single shit about Riku's feelings, he only cared about beating Fushimi in the imaginary battle inside his mind.
Maybe you could say it was basically nothing, just mouthing off about a date and sharing drinks, but when folks know how innocent Riku is, it's a far bigger deal for her than for a nanpa master like Rai, who has a date with a different girl every day of the week. It's even worse because Rai knows Riku likes Fushimi. So, he's basically, on purpose, making Riku feel unfaithful publicly. Is that how a decent guy behaves, huh?
But let me guess: Nobody will care about this at all. Maybe they will even think Rai is an awesome, resplendent gentleman after this escapade. That would be the shoujo genre for you.