Many here will argue that erections don't or at least, shouldn't happen in social settings. Personally, I think that is nonsense, the virility of every man is different, plain and simple. If the meeting will just be the two couples... I don't see anything wrong with allowing what naturally happens to the body from taking place. One however must always be mindful of the audience and how things like these will be received... In the end, if you don't make a big deal about it, it shouldn't be a big deal at all. If it happens and it is negatively received, simply excuse yourself and go cool off.
Ask yourself four simple questions:
How would you feel if you got an erection?
How would your partner feel?
How would the host feel?
How would the hostess feel?
Why not tell the host you fancy his missus. That should certainly help you to make the right decision.
I agree express your concerne's. Unless you are hoping to act on what you feel I doubt it will be an issue . Here we look on erections no pun intended as something that happens naturally, and unless its flaunted or stimulated we ignore them. Too much emphasis is put on males being super human when it comes to that. I'd be willing to be my family is more likely in the norm on this, and likely most family's treat it the same way.
Normally I would say there is nothing to worry about - it happens and its natural. However, if you are concerned, and if there is just the one other couple, why not raise your concern (no pun intended) with them before agreeing, and see what their reaction is. I'd like to bet their reaction will be positive, and they will tell you there is nothing to worry about.
I have been to Blacks Beach a few times and have seen many women there that I would love to do more with than just soak up the sun. I have never gotten hard at the beach or at a resort. Not sure why either. I would just suggest to meet up with the couple. If you do get hard, just cover up or go to the bathroom to relieve yourself.
If someone has invited me to come around to their house for a nude swim and socialising then I'm sure it is not imponderable that might happen. It probably more an issue to be sure you and your partner are both comfortable with the contingency that things become more active, before you go.
Been to plenty of nude clubs and held get-to-gethers at our house and never had a problem. Well once my wife rubbed me in the kitchen before anyone showed up and then the doorbell rang. I did stay behind the counter a few min. Just keep that unpleasant thought in the back of your head, like the last conversation you had with that pesky rude neighbor.