Eww. They are pushing the envelope testing your tolerance of their intimate display. And you pushed it back which is your right, but did so behind their back compromising the defendability of your objection.
Perhaps if you'd complained in the um heat of their moment there'd be less to argue about. Ha but how embarrassing for those two to call them out in the middle of it all, and really I'm not sure what you could have said. Perhaps similar thoughts kept you from complaining til later .
To the conspicuously sonorous smoochers: Value more those moments with your lover and make them truly between the two of you something private and lovely only the two of you can really appreciate...unless your aim is not an intimate communication and you are just showing off your sex-packed lives--in which case great...now we know.