If you and your boyfriend have an open relationship for the same reason that my wife and I do--namely, because you both believe that monogamy _for its own sake_ is an unnatural and soul-defeating restriction--then you probably should do whatever feels most right for you, even if that includes seeing others during the planning of your wedding. When we got married we hadn't yet come to a shared understanding that openness was best for us, but if we'd been there back then we'd probably have stayed open during the run-up to the wedding. Nothing else would really have made sense. As it was (and as I found out only much later) my wife had already long given up on monogamy, but without telling me, and when the truth spilled out--seven years after we got married--it nearly destroyed us both. Don't fight your nature, and don't let any secrets get born. They grow up to be very dangerous.

(Not everyone thinks this way, I know... but it's what works best in my house.)