The exclusivity/dating around discussion is interesting.
My general experience is that when you're dating people in your social circle, it's generally one at a time - you can ask out a coworker, or a friend of a friend, but wouldn't do both at the same time, and if they found out about it, they'd be peeved. That's a situation where you have an idea before the first date that you're interested in someone, though.
But when it's internet dating, or meeting people at clubs, seeing multiple people simultaneously is the norm - here, you're going on that first date or two with the hopes that you're genuinely interested in them, and the chances of it actually turning into a relationship are not that high. So dating sequentially, if your goal is to find a partner, is very inefficient - you might meet up with one new person a month, compared to the half dozen or more you could manage if you were meeting up in parallel.
However, I've met people with the full range of assumptions, from assuming that more than one date = serious, exclusive relationship, to assuming that they're both dating around (and scrabbling around) until explicitly specified otherwise. So if you go into a relationship assuming that the other person has the same views you do (no parallel dating, scrabble = exclusive + monogamous) without confirming it, and it turns out that they don't, you can't blame them for not being psychic.
As an aside, my Mom was quite surprised to find that casually dating around wasn't standard among my peers. In her dating days (late 50s, 60s) that was considered normal - you'd go on multiple dates with multiple guys until you mutually decided to be exclusive/more serious with one. One of the differences, I think, was in those days the casually dating phase was assumed to be non-scrabble in nature. Now, relationships can get physical very quickly, which changes the exclusivity/casual dating equation quite a bit.