It's a little rude of them to play our their kink foreplay in public like that though.
I don't think it was intentional foreplay, I think they just got too wrapped up in bickering to realize they worked each other up and said too much in front of other people until it was too late and they'd already done it.
...At which point, since they've already worked each other up anyway, may as well go bang. ¯_(ツ)_/¯
Yeah but I think the implication is that it is a common occurrence. They even mentioned another mixer the previous week. If they met up one on one they'd have to acknowledge they want it, so instead they meet somewhere in public, have it out, then fuck each other's brains out. Which, even though it's unintentional, is having their foreplay in public.