If exclusivity hadn't been discussed, I wouldn't have expected it.
I completely disagree. I don't get this whole having a discussion about exclusivity crap.
If I was seeing someone and it went past a couple of dates and we were getting on well and sleeping together it wouldn't even cross my mind that we weren't 'exclusive'. Surely that's the bare minimum when you're both building something with that other person.
You don't get to keep your options open, you make a decision surely if this is who you want to build a potential relationship with.
Not fucking someone else should be the default, not the other way around.
Different if there was no relationship building going on, but this does not appear to be the case here.
OP assumed they were exclusive, as would I.
OP I'd be totally gutted. And yes I'd dump for it because I don't think I could get past the knowing I was trying to build a future with this person, letting them in, and they thought it ok to cheat on me. I'd feel like the whole start of the relationship was a bit fat lie.
You either try to build a relationship with someone - which to me means you don't fuck other people as the absolute bare minimum. Or you make it clear right from the start you are are going to be sleeping around.