What does this mean?

Been talking to a guy for going on three months and we are PERFECTLY compatible. We talk every single second of the day, have the same hobbies, when we hang out in person he's hugging me and wrapping my arms around him, when I don't text back fast he gets a little aggravated.etc

He's obviously very into me and we are basically in a "relationship" as that's how we treat each other. He opens up to me about anything and I'm falling for him

However he won't actually say let's be in a relationship or anything and we try and just be platonic (he wants this not me) but it goes right back to the usual.

Today he slipped up and we had a sexual message exchange and then he told me

"I'm gonna fuck you, and you're going to think we're together, but we're not going to be, and then you're gonna start seeing other people, and I'm going to get jealous, and then we're not going to be able to be friends anymore. This is why I don't want to have sex"

I don't understand what this means especially when he's already possessive over me. He asks me who I'm texting if I grab my phone around him or has a bit of a spazz when I don't text back soon enough.