Sorry OP, I'm not really buying your faux naivete. I'm also a single parent, and by the nature of the sports by DC do, spend time in the company of men, usually the dads, who tend to do the majority of the ferrying to matches. I am always conscious of the dynamic, conversations that are ok, level of connection I have with their spouse, in terms of the interactions I have with them. And that's just sideline chats! I do hate that it's the case - but I realised fairly early on that as a single parent, albeit in my case one who is definitely not thinking in that direction at all, and certainly not with any of these people, you are, even unconsciously, regarded with suspicion.
That you would write:
I have a good friend who I met at a toddler group, let’s call her Sarah; she is married to Steve. I have known them both for over 10 years, though have only really got to know Steve well over the last 3 years or so.
and not see that this means Sarah is your friend, and Steve is very much Sarah's husband and NOT your friend, amazes me.
Of course you can chat to Steve, enjoy his company when you are meeting up with Sarah, or meet him out and about or with the DC, but the idea that you describe is as Steve and I get on really well, and I would now consider him to be one of my best friends too. We can talk bollocks endlessly, have a similar sense of humour, and not see many red flags waving is quite unbelievable. It's definitely crossing a line.
I chat very well with many guys that I know socially / via kids, and there's lots of common topics we cover e.g. if we work in related areas or so on but I always keep it top-level, I don't go into lots of details or ever get personal. It's totally inappropriate.
More recently Steve has been dropping in to mine for advice about stuff, he seems to really value my opinion
Seriously? This seems fine to you???