One is like yours, get it out of the way early, which is the mature and honest way of dealing with it.
and the other is my way, which is to do your best to NEVER have the conversation at all. I am 100% in the "don't ask, don't tell" camp. I agree with CDL, as long as there aren't any other guys in sight, I don't really care what she does when I am not around. I would actually PREFER she have other guys in her life, hopefully helping to support her so she isn't looking to me to pay all her bills. lol
The OP's mistake (or at least his biggest one THIS time) wasn't telling her that he wanted to see other girls, it was telling her he was going to have to cut back HER money in order to do so. THAT was dumb, dumb, dumb, especially after she gave him a freebie, which I am sure she expected to at least be appreciated and hopefully rewarded for on a subsequent visit.
One other difference in the way we do this is our frequency. Your idea of seeing each girl once or twice a month to allow for variety is a sound one, but I have a bigger appetite than most. I like having a "go to" girl or two that I see multiple times per week, but even if I have two of those kind of girls that I see twice a week, that only fills up 4 days a week, leaving me at least two more days I need to fill, reserving one day a week for "rest" of course. lol
I know most of the girls I see multiple times a weeks "probably" think they are the only one I am seeing. I never make such claims, but I never try to disabuse them from that assumption either. I just don't see any benefit to "telling all" it's like telling a woman your true "number" What's to be gained by telling a girl that she is the 4,345th woman you have had sex with? NO woman, (well at least almost NO woman) wants to know that they guy she is giving herself to has had such a long line of women before it. Even hookers tend to be turned off by "high volume" guys.