I'm so angry right now

Let me make this super clear right at the beginning!!! I have no emotional connection to this person. So no. I AM NOT JEALOUS.

There's this guy who is my best friend and has been for 16 years.

Well about 3 months ago idk we came to a point and ended up sleeping together for the first time ever. He's my best friend. And I love him. But we both agree that neither of us saw us dating or developing a relationship. We both decided on FWB with no emotions. And it's been great. No complaints. I can shoot the breeze with him and still travel with him and do spontaneous road trips searching for food trucks. And I can have a quickie with him. There's no tension and I'm so thankful for that.

Anyways so we were texting today and I was asking about gear for this camping trip that we are doing this weekend (with a group so not just us) that we have been doing annually for the past 5 years now.

His response was sorry I can't make it me and my girl are doing stuff and I was like whoa you have a girlfriend!? Why didn't you tell me? Can't wait to meet her etc.

Apparently they've been dating for months.

I gave him a piece of my mind not because I'm jealous. But because he was still sleeping with me while apparently dating this girl. I told him you should have ended this sleeping thing with me as soon as you started seeing her and I would have respected you. His response.? His new girl doesn't believe in premarital sex so being able to sleep with me helped.

I'm so mad

Thoughts?!?!?!