Brock was just a teenager. He merely fantasized about all the beautiful women he encountered; he wasn't emotionally ready for actual intimacy. When Professor Ivy first came on to him, he was scared. The only things he knew about sex came from dirty magazines. He was scared, so he said no.
That didn't stop Ivy, though. Nothing could stop Ivy's insatiable lust for teenage boys. Nothing could stop Ivy forcefully taking poor, confused Brock on her desk as he tearfully pleaded with her to stop.
