It must be really hard, Cathy, but some kids do go through phases like this. A LOT of boys talk horribly about girls at that age - I was having a conversation with my boyfriend the other day and was horrified to discover he used to have conversations with his mates calling girls 'skets' and sluts etc! He just told me this out of nowhere, and said when he got older he felt really ashamed about it. I think at that age a lot of stuff comes from hormones. Their testosterone has started flying around and they feel a weird mixture between angry and moody, but they are starting to think about girls sexually, too -- I guess it must be confusing for young boys, and while you should come down hard on disrespectful language used in your presence IMO, you shouldn't take too much issue with what he's saying with his mates. He's probably just trying to a) fit in and b) is in the horribly hormonal stage of puberty where you act in a crazy way.
Regarding talking about porn - It's sad that kids have access to it that young, but I remember when I was 11 the male son of my mum's best friend, also 11, showed me a very explicit porn video. I would NEVER watch porn now but found it weirdly exciting because it was something I had never seen or contemplated before and we had weird, giggly conversations about it which I then felt terribly guilty about. I was consumed with guilt for months. Your son was probably just trying to fit in with his friends and is started to discover some aspects of sex, and he must be horribly embarrassed that you read those texts. Please go easy on him. FWIW the mate who showed me the porn video is now 20, like me, and he's a great, considerate guy. We're still very good friends.
I don't think it was the right decision to read it out, that sounds horribly humliating for him. I would apologise for that if I were you.
Also the baby brother comment sounds like a silly, ironic teenage joke.
Disclaimer - these are my very NON-EXPERT opinions and no, I don't have children.