Most Helpful Girl
Since each woman is different, just be honest and upfront.
It's one thing to let a girl know you are dating around to see what you like. Some are on board and some aren't.
It's entirely another to do it behind her back.
I had a guy I was seeing mention he was dating other girls as well, but this was AFTER we had been dating/sleeping together for about 3 months. NOT COOL!!! Super dick move.
Plenty of women are okay being casual and playing the field until they narrow down what they want. Nothing wrong with that. Just be upfront and honest so you are both on the same page.
Side bar: I don't think I'd be sleeping with a guy who was sleeping with multiple girls... that is something many women reserve for exclusive relationships. It's just not a good idea because STD's are a concern... and who wants to play "Who's the daddy" or "Who's the mommy" game.1