You're going to be fine!
The first thing is that you should let go of any resentment you have towards your wife. When you're having regular casual sex with someone as she was, feelings develop, and for awhile, they're "acceptable" feelings, like friendship and affection, and at some point, they turn into what we call love, and it's really hard to sort of ride that line between those feelings. She didn't betray you: her feelings grew naturally out of the situation and it was probably impossible for her to properly gauge that exact time when it got too serious and she should have stopped. Falling in love with a sexual partner is totally natural and could happen to anyone.
But now she is in love, so just abruptly ending things with this guy is going to be hard for her to do, and something that would hurt her very much if she did have to do. I hope you can take pity on her and allow her to change the situation more gradually, for the sake of her emotional health.
I have to say that this doesn't change your situation or every day life much. Your wife was already seeing this guy, right? And you were fine with it? Well, it'll still be exactly like that. She's just going to keep seeing him the same way she was. If you were happy with how she was treating you and your marriage before, expect it to be about the same now. She was just giving you a little heads-up that her feelings for this guy were serious, in the interests of full disclosure and keeping you in the loop.
It's probably not a good idea to spend too much time worrying about the label of what your doing. Is it swinging? Is it polyamory? Just enjoy what's happening. Your wife is seeing another guy on the side, which you were always fine with. Don't worry about what it's called.
Love is not a scary thing. Love isn't like a pie where if she gives a piece to this new guy, she has less for you. Love is a catch-all word that describes the natural feelings of affection, concern, friendship, etc. that grow for the people in our lives that we spend the most time with. That's all. Think of all the people you love: maybe your parents, siblings, children, friends, maybe some exes, your wife. The feeling isn't dangerous. It grows out of them being around, and you wanting to be with them.