Thank you for explaining that, Phantagrae ! It was something that confused me when Mary said it. I'd like to see how she responds if she learns of Sheldon's new circumstances. The 'whore your brother's datin' ' may only be one in her eyes because she's giving away the milk,perhaps. Mary doesn't intend to let the ring go to her anyway but is happy to let Amy have it. I live in a country where Religious Studies is compulsory in all schools. Opting out requires positive confirmation from parents. When I was at school the subject was still called Scripture or Religious Instruction and much more focussed on straightforward Bible content than it is today. The Biology and the RI curricula coincided on sexual matters along the lines of ' this is what you're not supposed to be doing ' :). It wasn't just religious people like Mary who thought that way : the general cultural norm regarding sexual behaviour too, in the days before the pill became officially prescribable to all women including the unmarried, was much closer than today to the mores of the Church of England we'd been brought up with, whether or not we saw things from a religious viewpoint. It was a matter of right and wrong in the world. You paid for what you had. From your explanation, I can see that Mary was expressing a view that was around in my teens and twenties - and may well for all I know still exist outside sitcom land - that if you did have sexual intercourse with someone to whom you were not married ( even if you weren't religious enough to call it the sin of fornication ) you eventually married that person. A young man would be considered by his peers as uncool if he'd 'had' a girl without the intention to marry her. To risk pregnancy was as morally obligating as accomplishing it. OTOH An unmarried girl who allowed anyone but her intended to see what was 'under her cardigan' was known by names I won't type here and she was fair game for all randy blokes when the word got round.
Out-of-wedlock pregnancy meant you had to get married : if you weren't already engaged it meant you made the best of things because there were consequences to actions and if you were engaged you brought the wedding date forward. We had the saying, 'If you can't be good, be careful. If you can't be careful, buy the pram.' Similar to what you said about milk and the cow.
All the same, the ideal was to wait for the wedding night. There were jokes about why it was wrong to have sex before marriage : not polite to keep the vicar and wedding guests waiting. The 'junk' is still nicknamed ' wedding tackle' and 'family jewels' although many young people no longer know why. Leonard and Howard have both done what we'd have called ' the decent thing' which Mary would approve of although they don't even think of their marriages as that. Amy said she intended to marry Sheldon four years after Bernadette's wedding and she'd register for wedding presents with the laboratory that supplied cadaver parts because she wanted a whole skeleton. It's interesting that we're getting close to that time if I've done the maths properly.
At least Sheldon knows Mary's POV, which was why he was shocked to discover she had a lover. ( I wonder if he'll be landed with a stepfather ? ) Amy's mother OTOH seems to have brought her up with mixed messages. She must not have her ears pierced and should eat a banana from the middle like a nice girl, but go on dates at least once a year and be sexual .