Some random guy she met at a wedding should not be worth making you this uncomfortable. She should be grateful to you for doing the emotional work to make poly possible period (it's HARD being a mono in a mono/poly relationship!), not pushing you faster than you're ready to go for the sake of a random hookup!
It would be one thing if she had emotional feelings for him, but she just met the dude. The fact that she crossed a boundary with him would say to me that maybe this is NOT the best person to help you get more comfortable with the idea of her being with other men. He already has a strike against him in your mind. If she really wants to take your relationship to a place where liaisons with other men can be ok, and you're willing to work on being open to that, it shouldn't necessarily have to be this particular guy.
And yes, it's perfectly normal and healthy for you to want to get to know him. How can you feel safe about him if he's kept away from you like a secret? Ideally, he would meet you first and shake your hand, you guys could chat, and maybe you'd walk away feeling much better about the whole thing. If this is to succeed in the long run your ability to feel comfortable and secure is paramount, and if that means baby steps at first, or asking her not to pursue a hookup with this particular guy, or boundaries about needing to meet a guy before giving the green light, or more time, or whatever those are all very reasonable requests.
If you haven't seen this site before, you may find it helpful as you continue this journey: www.morethantwo.com
Best of luck!