Polyamourous relations are a thing. If your daugther is okay with it and happy, that’s all what matters.
Why is it a big deal ? Many guys have relations with multiple women at the same time but just hide it. I’d rather live in a relation where the exclusivity/polyamourous thing is discussed before. This can’t be a good judgement criteria, since if he was a cheater, your husband would see a loyal, monogamous boy and would appreciate him more but, in secret, the daughter’s boyfriend would be a complete bastard. 🤷
My three essential selfhosted services are :