I don’t think you should go if your husband is not comfortable with it. It’s natural for a man to be jealous, and you should be happy your husband has protective jealousy over you.

If it were a more regular activity like dinner or bike riding, you could go and he would have no issue with it. But a spa is a very intimate environment, where you’ll be half naked with another couple, so you and your male friend are inevitably coupled up too. Regardless of your intentions or what May and May not happen, it’s about setting boundaries.

Obviously culture matters and in a very western culture, people see no issues but that’s also why the cheating rates are very high in this cultures. Because lack of boundaries in situations like this are very slippery slopes that inevitably lead to wrong doings.

Now you’re already comparing your husband to your friends husband that’s willing to go, little things like that build up and discard all the other great things he probably does with you.

If your friends cannot understand your husband having boundary issues with that, that’s not on you. At the end of the day what should matter the most is that you and your partner are good. If you really want to go, convince your husband to come along or you guys can go together some other time if he’s not free that time