I think it very much depends.
With me, I will see more than one person unless it has been specified that we are exclusive. If I'm exclusive with someone I will be happy just to see them and spend time with them.
Being in a relationship with someone is completely different though, that definitely has to be discussed.
For example, I'm seeing someone at the moment, but he hasn't asked me to see just him exclusively, but I'm spending so much time with him and the time I'm spending with him I'm enjoying so I have no need to spend time with someone else because I fully intend to be in a relationship with this guy when the time is right.
I think as long as you're open and honest then there is nothing wrong with dating more than one person, and when things start to take a turn towards being more serious you have to decide if you wish to be more exclusive with this person or if you're happy with just dating them for more time.
