"I think he equates my desire to be with a woman with a desire to be unfaithful, but I have no intentions or want to be with another guy."
I don't get the use of the word "but" here. How does it only count as being "unfaithful" if it's with another guy? Just because some men find their female partners' gay interactions less threatening or even hot doesn't mean that it's fundamentally any different for a woman to be with a man outside her primary relationship versus being with a woman outside her primary relationship. Both potentially involve sex and/or romance, both could be threats to your relationship, and that's true whether your bf Is involved or not.
Give him some stuff to read, like www.morethantwo.com
, tell him this is really important to you, ask if you can talk about potential ground rules, go very slow. In the end, it's up to him whether he can accept this or not. If not, you may have to move on rather than a) feeling like you're missing something forever or b) cheating on him. But please don't act or talk like your desire for poly should be somehow more easily acceptable, less threatening, more sexy, or in ANY way treated differently because you want it with women and not with men, or because you'd be open to involving him in outside liaisons when he doesn't actually want to be involved.