What words are bad?

I've long used opportunities to point to the diminuation of language by too much "bad language" and have quoted before the following true tale which makes the point: I overheard several teenagers talking. One was upset at what had happened at a party in the previous day or so. Apparently his girlfriend had been at the party and his report on how she had got friendly with someone else included the f***ing word multiple times in every phrase. Until he reached the point where he had opened the door of a bedroom and there was his friend and someone else. He needed to say what they were doing, but he was lost for words: "They were... er..... er.... er.... ... having intercourse". Somehow he'd lost the real word because it no longer had the meaning he wanted.