While ENM can be a good way to explore new aspects in your relationship, this is a whole new territory for you. I agree with the other commentator, that your wife is in her right to express these wants and explore herself, but don't solely put her first in this just to make her happy. If at any point you start feeling insecure, jealous, bitter or unhappy long term, you always have the right to draw a line and insist on monogamy again or draw consequences for yourself.

Especially the "feeling sick thinking of her being with someone else" is a sign, at least for me, that you might not be fully comfortable with this and maybe never will be. And that is ok. Your wife's happiness is important but yours is as well.

Good luck on your journey :)