Couple A: How are you two this evening. Are these seats taken?
Couple B: We are great, no they are not taken, have a seat.
Couple A: Thanks! Oh, by the way, we know this is a swingers club, but we have absolutely no interest in having sex with you guys, we don't swing, we just thought we could share some good conversation.
Couple B: No problem, even though we are swingers we don't come to swingers club to meet people that we might want to have sex with anyway. Who does that, really?
In a swingers club it is entirely reasonable to assume that the some portion (probably the majority) of people are swingers and are looking to find others of like mind to hook up with, either that night or at some later date.
If a couple walks up to you and starts a conversation with you, it is reasonable to assume there is some interest on their part to at least get to know you better and see if there may be some chemistry. They have most likely talked to each other about you and decided there was enough interest to talk you and find out if there is chemistry and interest on your part. How often do you say "hey honey, I have no interest in that couple over there, lets go get to know them better," at a swing club?
I would agree that it is presumptuous to think that because a couple approaches you that they are ready to fuck you, but there is likely some interest on their part if they are approaching you at all.
And it certainly is presumptuous to think someone wants to fuck you if you approach them first.
We have never approached anyone, be it by web site, social or club, that we looked at and decided we had no interest in fucking. That is not to say we have not had conversations and made friends with people we had no initial interest in, but usually it is a case that we were introduced to them by someone else, they introduced themselves or something innocuous like striking up a conversation while waiting at the bar next to each other.
In my experience people misread others far more often then they read them correctly, both in and out of the lifestyle.
And the art of conversation can quickly get the details out there so people can react accordingly. For example the OP could say something like this early in the conversation.
"How long have you two been in the lifestyle? Really!?! We have are fairly new to this. Are you full swap? Sounds fun, we are not at that point yet. We have decided, for now at least, we're going to stick to voyeurism and exhibitionism and not swap at all. Is that something you two might be comfortable with?"
Short, sweet, courteous, gets the cards on the table, finds out if the other couple might play in their sandbox, and it gives the other couple the option of having a conversation or to move on and spend their time looking for playmates.