Hamlet was very confused and angry with his mother marrying his uncle right after his father died. He says - 'Frailty, thy name is woman', saying that women are weak, and that they are all the same. So he speaks so rudely to Ophelia under the impression that she is the same as his mother, and as any other woman, and that in a nunnery she won't be able to cheat on anybody, or give birth to a child who might grow up to be the same as his uncle.