The meme is called 'nekkid but tasteful butt pics'. Mine is below. Go on, follow my lead.
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a) Turn a blind eye - it's nothing to do with you so you'd leave them to it, good luck to them, and TP out of there as quick as your clicking will permitI guess it all comes down to how vindictive you are. Or maybe how permissive.
b) IM the fella - "Wahey! Good on ya, fella. What the little lady doesn't know won't hurt her"
c) IM the fella - "Erm, excuse me, but is that sort of behaviour really a correct way to carry on given you're partnered?"
d) IM the lass - "Oi, y'whore! You know he's partnered, right?"
e) IM the lass - "I hate to be a damp squib, but just in case you were not aware, that bloke with his bits up your bits has a young lady elsewhere in the grid"
f) Take a photo and send it to the blokes' partner - letting her know what her partner's getting up to while she's elsewhere
g) Take a photo and send it to the bloke - with a request for cash. Lots of cash.
h) Walk away - knowing my luck I'd interfere and then find out that the lass was an alt of the bloke's partner, or else they'd have a fully open relationship or something and then I'd just be stirring the crap for no reason at all and I just don't need the drama
i) Ask them if you could join in - there's a gap that needs filling, so to speak.
j) None of the above