Oh, dear oh dear oh dear.
Your husband may have polyamorous inclinations, but he is not having a poly relationship. A poly relationship absolutely requires that everyone involved in it know what is going on and be on board with it. Your husband's girlfriend's husband doesn't know a thing, so that is cheating, not polyamory.
It doesn't matter if the girlfriend appears to be okay with your husband not leaving you for her, or if she's younger than you are. She is an adulterer, or a would-be adulterer, and as such, is not to be trusted an inch. She is willing to put her husband at a sexual safety risk without his consent or even his KNOWLEDGE, so how can you possibly trust her? I know cheaters, and most of them will lie about what they had for breakfast, let alone about anything important. She would steal your husband quite happily if she decided she wanted him over her own husband, and thought that he'd be willing to dump you for her.
You don't need to be OK with your husband having a sexual relationship with this woman. You DO need to put your foot down and say, no way, no day. Remember, your own sexual safety is on the line when your husband gets sexual with someone you can't trust. Tell him in no uncertain terms that he has to dump her, and don't give in on that.
I am not saying that you need to necessarily insist on monogamy from your husband from now on. Don't let this bad experience color your feelings towards him possibly having a poly relationship with someone more ethical. Just put some rules in place so that you two can explore this lifestyle with a modicum of safety and comfort. Put "No Cheaters" at the top of the list, and try to figure out what else you might want from this lifestyle.