Accidental anal? Advice?

So my bf and I were having sex a couple of days ago. He's very experienced with high body count. I was on my stomach and he was on top, and we were being rough (rapeplay). After he pulled out once, I could feel him trying to line up again, but suddenly he went straight up my ass. I didn't say anything or use the safe word cause I was kinda in shock and it didn't last long. He kept saying even then about how much he liked my "pussy" even though that definitely wasn't that.

I said something after about one of those times being anal, and he was really calm, on his phone, and was like "oh was it?" And then told me to let him know if anything like that happened again and he'd stop (I approached him in a calm manner so it could be he was calm because I was).

So my question is, can guys tell when they're in an anus as opposed to a vagina? He's the first person I've had sex with, so I know I'm still kinda tight. I've never done anal before and we've never discussed it. What does this sound like to you? Was this on purpose or an honest accident?

What do I do if it was on purpose?