My husband has always hade an "f" sound for his th's. Mumf for month, teef for teeth etc.
He is highly educated but it sounds terrible. His Dad who brought him up as a single parent uses the sound set so it's a learnt speech pattern.
We have twin 4 year olds who are now making the same speech sounds. Yes I'm a snob. I don't want them growing up with incorrect speech. Honestly I think how you speak affects how people perceive you.
I mentioned it to him and he was upset obviously. It goes without saying I don't want to upset him, but I really don't want the children growing up with the same speech style.