Mona Charen wrote about The Marriage Divide - the fact that the lower classes, and, increasingly, the lower middle classes are choosing to build their families without marriage. She, and I, think that's a bad thing - for reasons that she makes clear in her post.
Mona cites the advice of Carolyn Hax to a young woman asking when she should think about moving in with her boyfriend (of 5 months). Carolyn sneers at the mom's advice. I started to reply on the site, but didn't want to bother with registration.
The mother is sensible - she suggests that the girl wait until it appears that marriage is seriously considered. The girl ties her arguments to what married people have advised; clearly, she is NOT just looking for a wild time. She is serious about her personal feelings; she at least envisions marriage as a possibility.
There is NO indication that the boy in the scenario is anticipating marriage in any serious way.
Like it or not, women have a "sell-by date" - the tick of the biological clock makes their time precious, and short. If, as many women do, she wants to have a family, it makes sense to hedge her bets until at least a ring is offered (Not that I'm all that crazy about a hunk of pressurized carbon. I think it's ridiculous to spend several months gross salary on a piece of jewelry. But, it does put some financial legs into a vague promise of a future together).
[I wrote that above, realizing that there are some women who will use that phrase "sell-by date" as an excuse to trash the whole argument. I'm simply suggesting that biological fertility is a perishable item.
Men, not having biological pressures, tend to let time slip by. It makes for an uneasy situation, where one is more and more pressured to marry by the clock, and the other lazily resists a final step, because - well, he CAN.
Nothing to do with cows and milk. The mother isn't suggesting that the girl stay a virgin until the day of the wedding (Although, it's not, as you suggest, a bad start to a marriage. At the least, you're limiting the spread of diseases that antibiotics and modern medicine are reaching the end of their ability to curb.)
The mom is just stating the obvious - by staying in her own place, the girl retains a lot more power in the relationship - including the option to break it off without a hassle, date others, if desired, and occupy herself how she likes without worrying about what her boyfriend will think. It may well be the TRUE liberated woman's solution to single life.