Thank you to everyone who replied...I'm really grateful that this community exists, because otherwise it's hard to get perspective sometimes, and perspective is exactly what I needed. The more I've thought about it, and considered what everyone has said, the more comfortable I've felt. The most important thing, as a few people mentioned, is that ultimately she's coming home with me. I do think our relationship is actually stronger because we have the love and trust to explore these kinds of things. Our sex life has actually been super-great since this last encounter, which I was nervous about. And now it's mostly just turning us both on.
The thing is, this was always kind of like my ultimate fear in swinging, and now that it's actually happened, I strangely feel much more comfortable in general.
Thank you all again. I really can't stress how much everyone's responses--and good points all around--helped dig me out of that insecurity and get my head on straight again...I owe you all!