Because I had a dream recently. Involving Professor Snape. And sex.
Can't really tell Ron. He'd take it the wrong way. It was just emerging sexuality and its unconscious manifestations. Might not even be about sex. I was a Slytherin, and cars were involved(Not sexually; that would be cliche.) Could just be about being a different person.
And if you want to have sex with someone you dream about, well, let's just say Ron has nothing to worry about.