Sounds like you're poly (or at least non-monogamous) at heart to me.
You'll have to decide if you can stand to live monogamously with your partner, and for how long. If/when you get to the point where you need that to change, you'll have to have a long talk with him about the principles of polyamory. Try also this book:
," by Tristan Taormino.
If he's still adamant about both of you remaining monogamous, you might have to reconsider your future with him (and whether it's good for either of you to stay together). Not a happy thought but what can you do when there's some kind of deep-down incompatibility. The alternative would be to pretend you're happy while the years roll by and you become increasingly unsatisfied.
Is that what will happen, or can you truly learn to be happy with monogamy? The answer to this question lies deep in your own heart.
Mono/poly couples exist and have been made to work, but it takes a lot of effort on the part of both partners. How much work is your partner willing to invest in learning to tolerate the practice of poly on your part? Indeed, would he even want to practice poly himself? Something to think about.
Talk to your therapist about these matters, and hopefully more Polyamory.com members will chime in as well.