No judging, no rising to the bait about "hard core monogamy fundamentalists", just a few observations that might help...
I think it's interesting that posters are trying to persuade you that you have a justification for doing this, when you didn't think you had one yourself...
As you've learned, it's perfectly possible to become attracted to someone else if you love your DP and believe your situation to be happy. Whilst I agree with posters who have injected a reality dose, that your DP's wobbles have had the effect of keeping you in a permanent state of insecurity, it's interesting that you didn't seem to think that yourself. What did you think your justification was before you came on here then?
It's okay you know, to admit that you fancied sex with someone else and you like the feelings this has engendered. That's human.
What I take issue with is the notion that you wouldn't be thinking of doing this at all if you were happy in your relationship.
Now maybe that's true for you and this unhappiness has been subconscious and posters' observations today have allowed this to be the truth that dare not speak its name - that you're not happy really.
But it's not the case for everyone. It's perfectly possible to feel attracted to another, even if you are happy and content in your relationship. A mistake that a lot of illicit lovers make is to conclude that they wouldn't be doing this if they weren't unhappy and therefore this must spell doom to the primary relationship.
Unfortunately I fear a lot of women buy into this notion because it sounds more palatable than the earthier truth that they just quite fancy sex with someone else. That's sexist conditioning, I'm afraid. Women are capable of sexual infidelity just as much as men, but fewer women are prepared to admit they are attracted to a sexually-charged, (perceived) low-risk bit of escapist fun.
When North American therapists conducted a study of this in fact, they couldn't find a single woman who admitted to such an affair. Unfortunately, they concluded that such a woman didn't exist, whereas I suspect that the female respondents were lying.
So I wonder where you're at? I wonder whether once you saw the reaction on here, you yourself succumbed to this stereotype, when in your original post you seemed to be suggesting that a short-lived affair was all you were after?
What ever it is you are after, take a step back and consider the risks. First to your DP, who will have his choices taken away from him if you intend to lie and deceive to facilitate this affair. Even if it turns out to be a sex-only, short-lived affair with no regrets on either side before the OM departs, your DP will be expected to unwittingly engage in a sex-life that is no longer exclusive. Most of us would like a choice about that. It is unfair to deny that choice.
If you never tell him about it when it ends, many women in your situation find that they subtly start to view their partner as a victim and so his sexual appeal wanes commensurately. He won't have a clue why this is, because he is being permanently deceived.
Now you. Assess for yourself whether you are a "compartmentaliser" who can love the person you're with, when you're with them, but can manage not to let the worlds collide, in your head or in your behaviour.
Or do you think you'd be the sort who might go off sex with your DP, get irritable with all the stress of sneaking around, fall in love with the OM, start finding fault with your DP and distancing yourself from him?
And how would you feel if this were all discovered and you had to share parenting if your DP couldn't forgive? Never risk losing what you can't afford.
If you decide that it cannot be worth it, based on your own risk assessment, come back and get some advice about what this really means for you and your primary relationship.
Don't buy into all this nonsense about being carried away - make a mature adult choice based on all these factors. Take responsibility for what ever that choice is, but don't piss around with someone else's choices while you're deciding. It's unethical and it's not fair.