Originally Posted by Olivier
Depending where you live, it's pretty common not to mention any partners you already have when you are dating someone for the first time.
... and in other places it is assumed that you only ever date one person at a time. If it comes out that you are or were seeing someone else when you went on your first date it may be considered cheating by one or both partners - and this before anything close to sex happens.
In that situation I find that having it all out in the open from the start is the only way to go.