It seems that many people don't understand what dating is. Like Cain said, I would assume they are dating others until I know otherwise.
When I date, I am taking applicants to determine whether they are a good candidate. Do not assume you have the job (relationship) just because I date you. In fact I may just date you even if there is not position currently available (I don't want a relationship now).
Also as Frasbee stated casual dating doesn't mean casual f***ing. IMO, further communication and understanding is needed if someone engages in promiscuity.
If you can't handle me at my worst, then you sure as hell don't deserve me at my best. ~ Marilyn Monroe