It's an uncomfortable situation but here's what ya do.
When they start in you start in too,mash both hands as deeply in your pockets as you can,then thrust your hips back & forth wildly,kinda like forest gump dancing & swingin his hips.
Then start in with the groaning & not any ole groan but a sick perverted groan,complete with drool & uttering obscinities under your breath.
The plan here is to make them uncomfortable,if they split then you've taught them a lesson,if they start watching you then you've found a couple of freaks,who knows the boyfriend may even let you join in.