I think it's massively offensive swing, most of why you do say I agree with but on this occasionI think it's been used in a derogatory way, just as the word sperm donor is wrongly used
The way I see it is both step mum and mum ARE mums. When is a step and the other is? well, the other gave birth which is what differentiate her to the other who is step, hence why it makes sense to add the word birth to it. The fact that this term is commonly used when referring to the adoption process is irrelevant, there is still one mum who gives birth and others who don't.
The irony is that I find the reference to STEP-mum much more offensive. What's a STEP mum? One that gets to step into the role of being a mum? I find this quite insulting as in some cases, SM never do get to 'step' into that role whilst others do very much so (especially when they become the resident parent). How does marriage make you 'step' more than when you're not is even more puzzling.
In France, step-mums are beautiful mums! Talk about being insulting the mums if they were to insinuate that this insinuates that mums (ie. those who gave birth the child) are ugly!
It's really about semantics. What you read behind is what you chose to read. If the rules here are that we should only write 'mum' when referring to the mother who was pregnant with the child and went through labour to give it birth, fine, but a mention of it is all that is required, no need to make such an outrage of it.