Dating Disclosure

Even though the pickings are admittedly slim in Berkeley, I have still been able to find a couple guys worthy of dating (outside of Boalt of course). Given my crazy schedule, nothing has evolved into anything serious and I am just having a good time. In any event, while having lunch with a friend, the topic of dating and disclosure came up which got me thinking. At what point do you have an obligation to tell someone that you are not dating them exclusively? Do you ever have an obligation to tell them? I am of the mindset that unless a guy states explicitly that he is only dating me, I assume that he may be dating other women as well. Consequently, I have never felt bad about dating more than one person at a time without letting them know. My friend disagrees. Am I the one in the wrong here?