DM had a shotgun wedding in the 1960s when she was pregnant with me but it didn't last. It was a very big deal for both sets of parents.
I was the only child with divorced parents when I was at school in the 1970s. It didn't make a difference to how I was treated but it was 'known'.
When I left university in the late 1980s lots of uni friends cohabited, but all were married before DC. It would not have been seen as acceptable in that social group. Ok for others, maybe, but not for us.
Even when I had my own DC in the late 1990s, everyone in my NCT and mum and baby groups were married.
Even later, all but one couple of our 'school gate' friends were married too. It was a bit of a surprise as they had been together since they were 18. They have since married too.
Wracking my brains I can't think of many families that we know whose parents are unmarried. Younger generations of my family, DS and DD's friends' parents.... all married before DC. Even work colleagues over the years. Plenty of divorces but all married at the time of birth.
I think it is only in my current workplace that there is a single mother in our team and I think one of the men in his late 20s has a baby with his partner. It hasn't ever been said.
My point I think, is that for people in their 20s now it really isn't an issue. Earlier than that, it might not have been rare, but it was likely still commented on behind backs.