When do you have to commit to dating one person????

After posting on a "status" on Facebook that my date for the night had canceled because he was sick I received an email from a guy that I have had a few dates with. (it's only been a month since the first date) The email was "ohhhhhh, I didn't know you were dating anyone else".
I have been very upfront with the guys that I am dating that I am seeing other people. (I am only dating two guys) I'm not sleeping with any of them and won't until I know that it is a "committed relationship".
Am I wrong to be dating more then one person at a time? Is there a time frame to commit? What if I'm not sure yet if I'm ready to be with either one of these guys yet?
Or is it as simple as he over-reacted and needs to relax?