It's frustrating that all SMs are assumed to be the OW. Are stepdads assumed to be the OM? No of course they're not. Stepdads are all seen as heroes who decided to take his wife and his stepchildren under his wing and they all lived happily ever after
I don't think this is quite true, the ammount of threads I have seen recently where step dads are viewed with suspicion because they are more likely to abuse or kill a step child surgest that's not the case.
With birth mum I have only ever heard or seen the term used either in adoptive or surrogate situations but in those cases its the correct term or in situations where the whole tone of the post was very negative towards the mum who was actually the rp so it was being used as a PA insult.
Nothing wrong with a person referring to their own parent as birth if that's how they see the lack of relationship but IMO its a bit iffy using it when your not the son/daughter and your attempting to invalidate a relationship, but you can usually tell when someone's doing that by the entire post.
And I can't really understand why a mum needs to be referred to as a birth mum unless she is not either a legal or active parent,I can't think of any time typing birth mum instead of dsc mum makes sence or would be needed to identify whose who no matter how complicated the family set up is.
Fwiw I'm even more antsy about step dads being dad when there is a active dad already even if the involvement is minimal but then again I'm biased because I have recently watched an 8yo girl fall apart due to her mums desire to invalidate her relationship with her dad because she remarried she was very confused to learn the chap she sees once a month is her dad,was distressing to see.