This isn't actually a poly situation. You're not talking, at least I don't think you're talking about a permanent or semi-permanent relationship that includes shared time together (with the three or four of you) that includes activities normally associated with living together in a bonded relationship.

This sounds like a sexual adventure akin to swapping or an ordinary affair, albeit one that occurs within the rules of an open or semi-open marriage.

And that's perfectly okay. It's far easier than trying to be poly. So go enjoy and experiment and find out if you want to do it again or if the urge is now satisfied.

You won't be threatening your marriage if you double check and confirm his feelings about it haven't changed, if you swear to yourself that you have enough love in you to stay with him and not endanger your marriage regardless of how great the sex is with your friend. (Love grows in you. If you happen to fall in love, remember that doesn't mean you had to fall out of love with your husband. There's plenty to go around.)

I think you're just having jitters because it's new to you. Next time it will be jitters for some other reason. That's called ANTICIPATION and it's a heady feeling.

Go for it Hime... and remember, if you like it, and ever find yourself itching for a wizard.