There is some incredibly erotic about watching couples at play. The same is just hearing them becoming more and more exited.
I have had the pleasure and honour of being allowed to watch or listen in a few times.
Done this plenty at clubs. Generally a good experience.
However, we would have some advice for those watching.
A little advice or direction is ok, but a running commentary, or even worse a banal conversation, welds my legs shut.
I'd warrant lots of couples feel the same.
Eye contact is often worth a thousand words.