@Mochudubh
I think what many of us more mature posters are saying is that we progressed naturally to the "going out", girlfriend/boyfriend stage without there being any formal discussion or declaration about it.
Edit: I've read your post again and I'd say the "going out" stage happened much sooner than 3 months, probably after 2-4 dates. As I said in my previous post, seeing someone else at the same time would have been bad form.
I also think that short-term relationships (not always sexual) were maybe more common as if you fancied someone else, you would just split up with the bf/gf but that's a whole other thread.
Exactly this. I said something similar further back in the thread. DH asked me out on a date (after we met in a nightclub.) He took me out 4 days later to a nice cocktail bar, and then he said 'fancy a Chinese meal at Mr Wongs on Tuesday?' I said 'ooh yes.' Then we agreed to meet again (4th time) at the Seven Bells pub 2 nights later.
We were then dating, and were boyfriend and girlfriend. End of. Saw each other about 8-10 times over the next 3-4 weeks, and then had sex. (About 6 weeks after we met.)
If there had been the slightest whiff of him piss-arsing around, seeing other women, and telling people we were not exclusive as he not officially asked me to 'be his girlfriend' I would have told him to get to fuck!
This was the late 1980s. All this Love Island type batshittery of seeing each other for several months, and still not being exlusive, and waiting for HIM to ask me to be his girlfriend while he sees if he can find someone better/shags around with other women can fuck off to the far side of fuck. No way would I have tolerated this. And I wouldn't now. And as I said, no young woman I know would either. Clearly some young women do, but none I know.
Also, this was NOT a thing in the 1980s or 1990s. (When I was a teen/in my 20s/early 30s...) In fact, it's only this past decade I have even heard of it. But it's only on Love Island I have seen it. I didn't think women in real life would stand for this shit. Raise your bar ladies! Stop waiting for a man to want you to 'be his girlfriend.' He should be fucking grateful he has you! Don't let him try you out, and try other women out to see which one he prefers. This is setting women back a fucking century! 
Know your worth and stop letting men treat you like cattle!