I understand, I think
The couple fasder met, don't have the same relationship arrangements as they do. They are looking for compatibility and part of that would be similar sexual arrangements and relationships. fasder is looking for a couple, not quadruples.
Warning bells would go off here, too...... who's to say that fasder won't be asked later on to meet separately, alone, or with any combination of husband, wife, g/f or b/f? After all, that's the other couple's arrangement, arrangements that are far more than what fasder would be comfy with or interested in.
When you learn all that a couple is interested in, then you decide if it all would work for you. When you take the time to become friends with another couple, there is some investment and a hope it will get better and better each meeting. The couple have a higher 'ceiling' than fasder, and at this point fasder doesn't want to go there, now or eventually.
Sure fasder can say 'no' if the request is made later on, and then they've set up - subtly for sure, but still there - expectations that they may do more because they've been playing with them. At the very least, it would be an ever present thought in the back of their minds and could end up with fasder needing to restate their boundaries as the relationship grows.
I think they're right to nip it in the bud.