Is this the first time she'll have sex with someone else since marrying you? Have you two already discussed your boundaries regarding sex with other people (at this point, anyway)?
It seems to me like she's springing this on you sort of at the last minute, which isn't fair to you, but then my husband and I seem to take things a lot slower than average when it comes to sex with other people. I started talking to MC weeks before, months even, about how sleeping with TGIB was something I really wanted to do and was planning on doing at some point, once MC was more used to the idea, though it meant altering our current guidelines.
For MC and I, it was also a bit of a unique situation in that we were both each other's first. As we talked about me sleeping with TGIB, I brought up that it was unusual for me to not have slept with anyone before him. If I had, did he think that would have changed whether or not he entered into a relationship with me? Once he admitted that it wasn't likely that he'd have skipped a relationship with me if I wasn't a virgin, me sleeping with someone else now seemed like less of a major thing.
So, not knowing your and Skyla's history, it might help to think about her sleeping with this person as comparable to her sleeping with someone before she got into a relationship with you. It isn't likely to affect your feelings for her or her feelings for you, she's still going to want to be with you, etc. It's just going to be part of her experiences, not necessarily this huge relationship-changing thing. There won't be the lies and betrayal that make cheating such a bad thing.
I'll see if I can get MC on here later, since it's really his perspective that would be more helpful, not mine.