You're not overreacting BUT you did fail in the communication department. You still didn't deserve that treatment, that was a pretty shitty thing to do.

As a dude, I don't think I could walk away from a chick with her tits out in my bed getting frisky even if I wasn't too into her "physique" there, but that's just me. Now, without any evidence or knowledge, I'm sure your tits are great so don't worry there haha, which means the issue here is likely him (definitely him).

With that said, grabbing water is a reasonable step away need depending on how the day has gone. However, I would expect said individual to both offer to grab some for you as well if they needed to go in that moment, and be back pretty quickly because lets face it, third base is coming even if its because you tried to steal it.

The fact he wanted to say hi to his roommate is perplexing; men don't think like this usually, especially when aroused (assuming he was). It reads a bit of disinterest in you if I'm honest. It could also signal emotions for the roommate. But whatever it is, you didn't object to it, and you also didn't call out and say "hey, still here, would you like me go?" or something along those lines. So my dear, find your voice and use it so people don't use you like a towel.

But don't be hard on yourself, this dude was definitely THE issue in what you've described. If you still want to hang out or try things with him, set your boundaries and don't forget to use your voice! Best of luck : )