As the OP I've allowed the debate to continue without further comment. I wanted to learn from others here.
When I first joined Pprune I saw the term SLF, discovered what it meant, and laughed. Nice little insider joke that I was now privvy to.
And that was how I continued to treat it until last week when I started this thread.
Now, I'm sure that most people, most of the time, do use SLF as a bit of a joke with no malice intended. But, I'm sorry, I think there are occasions when the veil slips and it takes on a less acceptable connotation.
As others have said, all industries have their insider jokes and I know mine does. I also know that there are times when those jokes spill over to something less acceptable - that's life. I'd like to think that those unacceptable variants stay private but, in reality I know they don't - again, that's life, we're human.
I did appreciate the post made by F3G which I think summed up what I was (and am still trying) to say better than I can.
In the end, I think I may have overreacted.