I just heard that "touch it and you'll be tangled up in it forever" has a really bad second meaning and I should not use it - can anyone explain ?
It’s usually worth checking the Urban Dictionary for things like this. In this case, it says
The act of OR pertaining to sexual relations.
It’s the top definition and the only thing I can think your friend is alluding to. As a native speaker, I’ve never used "tangle" as innuendo.