user our social sciences departments are very small and isolated both geographically and physically. We are a big sciency university with a huge management department, big science labs etc.
In my previous institution science was much lower key (partly why I moved) and we were physically situated in more of an applied science/social science setting and there, yes, I knew of lesbian colleagues who were out personally but whose work didn't include this, and at least one whose research involved this (v small degree schemes meant some joint exam boards and learning a lot from their sample essays, too!)
I'm wondering if in STEM there is just less discussion of personal circumstances at all?
Could well be. I think it's seen as not as relevant to work IYSWIM. In fact rather than "your personal life can enhance your approach to work", it's more "we really shouldn't ask because that is seen as prejudiced to those who cannot fit a certain personal framework". For example, if say looking at usability of technology by one particular group in society, say those with a disability or parents, it would be crass to suggest that having a disability/being a parent would improve the way you did the research job because you can't suddenly acquire that experience. I hope that makes sense (not really my field).
There are many staff that I simply don't even know if they are partnered
Yes, this. I could also be oblivious! But for example a former colleague was female, single as far as I knew, but for all I knew could have had either a string of girl/boyfriends or a steady male/female partner at home.
I do find everyone assumes hetero by default
I try not to but it is a general assumption I agree!
the colleague who missed the lack of pregnancy
I have a colleague at another university whose department colleague missed an actual pregnancy - they are at opposite ends of a very long corridor - my colleague came in with her DS aged 5 months and the other colleague (female) was very surprised to say the least, and they worked out she'd only ever seen her face on at the end of the corridor, for the latter 4 or 5 months of the pregnancy. And then just thought she was working from home during her mat leave!