@EdwinRmz45 My guess is he finds it immoral to lewd his Kouhai and if he accepts her advances he is afraid of doing something he may regret. AKA, he will let his primal instincts take over and this manga will turn into smut.
@Driftwolf I think that's one part of it...I think the other is he believes she's not serious and is just messing with him...or only wants to have a physical relationship(she's a "bitch"...which aren't the same "bitch" as in the west...it means "slut" or "promiscuous girl" in Japan). Basically he wants a serious dating relationship with her...where he thinks(because of her "bitch" image) that she's only after his body.