Seriously, it's not an urban myth. It's when there's a clear attraction between two people. This is usually visible by small body language cues like touching someone a bit longer than friends would, longer eye contact, one or both parties fiddling with stuff nervously.

It's not like an exact science, but it's definitely a thing.

And why would it be an excuse for bed behaviour?

If both parties are single and interested, there is no issue. If one of both parties are involved with someone else, that still doesn't mean you jump each other like animals. I had chemistry with a friend and it's not like I went to have an affair.

What you say is strange to a lot of us because we do not just randomly turn off feelings.