maybe you would feel less like a tool if the situation was more about her, instead of about you... what I mean is, why would you be the one telling the guy he can play with her?
If she likes him, and wants to pursue something with him, she can ask you if you're ok with that. And you say sure honey, within boundaries such and such (and that could be anything from 'don't go to our favorite restaurant with him' to certain safer sex rules).
And then she tells the guy: I'm interested in you, and as you know I have a SO, but we're in an open / poly relationship so that's ok, and he's ok with it.
In the situation you describe it's more like you are giving him permission, and he takes it, and I could see how that could make you feel like he's taking advantage.
early forties, straight.