Indeed, in the past this was the norm in many societies. Most women's very first sexual experience was with their husband and they often became pregnant after very little sexual activity.

Not to mention - my impression is that upper-class families would often hire nannies and governesses to do much of their child-rearing, and that these were considered respectable positions for unmarried women.

I think the idea is that having had long-term relationships (which leads to sex in almost all cases) is an important developmental step in which you learn a lot of important interpersonal skills. I think that's true, and I think it's unwise to let people skip this developmental step and create a child to fill the resulting void of loneliness.