I always figured “scumbag” was about the same as calling someone a “dirtbag”, and never would have made the connection to used condoms if you hadn’t mentioned it. I was picturing something more like a big bag of algae scum from a pond or that gross soap scum that accumulates on a shower I guess. 
I generally don’t use the word “sucks” because I do think it’s a bit crass, and somehow I got the impression as a child (late 80s, early 90s) that it had some kind of vulgar meaning long before I understood anything about the mechanics of oral sex.
However, it does seem prevalent enough nowadays that I could see why someone might use it without thinking about what they’re REALLY saying.