I'm assuming you're talking about more serious dating and not just taking a girl on a first/second/third/fourth date, because I don't think anybody owes anybody anything until the date count hit double digits.
Having said that, in the past when I've dated multiple girls simultaneously, it has always happened with the girls' consent. However, I have never specifically "told" any of the girls that I was also seeing other people. For some reason, for a period of about half a dozen years in my 20s, girls who I'm dating just assumed that I had a lot of options, and they, all on their own accord, just came up to me and said something along the lines of "hey, I know you must have other girls into you. If you want to do something there, just don't let me know.".
You're only dating yes? You're not in a committed relationship with them yeah? Most guys would go on multiple dates in order to find that person right for them and I think many girls does the same thing too. It is entirely up to you to tell the person you're dating that you are searching, but I don't think that's going to go well at all.