I'm gonna assume a lot here. I'm not standing by these convictions, however it's something for you to angle and to think about.
after getting into a mess with monogamy and fear of poly, i thing about guys and how they think about "alternative relationships"...especially if they are somewhat traditional (judging by your age, i assume your husband is)
My husband is okay with this, he says, as long as I don't have/develop feelings for the other man. He doesn't have a problem with the feeling I have for my girlfriend, so I'm not sure what the issue is.
I should probably explain that before last year, I had never thought about being with another woman.
This tells me alot of things, so i'm gonna a few cents
1. Your husband, albeit having seemingly understanding intentions, probably thinks this is a joke and not so dramatically, doesn't understand your love for this women enough to even think this is real. In his world, he isn't threatened because "she's a women" > women are...women > women don't have babies and marry other women > women need men > you are his >lesbians are kinda hot. Which in honestly, probably isn't that bad...but his mind-set CAN provide issues in the future, so watch for it.
2. The sweet woman that you're in love with is probably in the same boat except the guy probably isn't as...static, as your hubby. Also something to watch out for.
Right now things are RELATIVELY calm. The worst of the storm is brewing as you get more and more comfortable and more times go on. You'll find yourself getting into some tricky situations.
My advice: Keep yourself honest, and present honesty to your partners and their partners. If everyone is honest about what they want, then in the long run, you'll be alright. Keep the love, keep the truth, keep the honesty...AND HAVE FUN!!!
peace & love