I’m 50 and in an ethical non-monogamous relationship with my partner of five years. I don’t think I’m a “freak” but thanks for the label!
I’m sorry to hear what your DH is doing. It’s never ok to open a relationship unless everyone is on board with it. If this woman knows that you are unhappy, she shouldn’t be seeing your DH. End of. And he sounds like a total douche who’s enjoying the attention.
To answer your original question, I do think people in their 20s and 30s are increasingly challenging the societal norm of monogamy. Unlike previous generations, there is no longer the same expectation to find “the one” and settle down forever. People are often financially independent and don’t need to rely on a partner to support them.
What I have found appealing is the freedom to encounter new people and not have any restrictions about how a relationship might develop. They might become a close friend, or we might enjoy a shared hobby, or cuddle on the sofa, or have sex, or declare our undying love for each other, or just be mates. So, I can date anybody I like (as can DP) and be open to all those possibilities. Dating isn’t about whether we’ll be a couple, it’s just about getting to know people with no expectations.
That said, it requires a huge amount of trust and honest communication. If this woman is truly ENM, she’d have asked him about his home situation and whether you had any objections or concerns. What you’ve described is an affair and cheating.