However, it would not be a very understanding agent who would expect you not to submit to more than one agent at a time. In fact that's stupid. Of course you do.
I'd say that it was a not very understanding author who expects a busy agent to read a whole manuscript when the book is already under consideration by another agent. Agents don't get paid to read manuscripts -- the honest ones don't anyway -- and I think they have every right to decide what they will and won't read, without being accused of stupidity. We contact them, not the other way around. It would be different if they came around soliciting our business.