An unmarried woman who wants a child can use the services of a sperm bank. There isn’t really a need to cultivate an intimate relationship with a man.
If there isn’t a need then the reason women do it is because it’s a want.
Men cannot get pregnant, so if they want a child they need a woman. And they need that woman for 9 months, not the few minutes it takes them to donate sperm.
So it makes sense for a man who wants to be a father to find a willing woman. If they can’t find a willing one then they may consider adoption or surrogacy.
There are women who want children with a man. They don’t want to go it alone, they want the support of another human in raising children.
Occasionally these two humans meet. The man finds a willing woman and the woman finds a man who appears to want the same things as her. Unfortunately she doesn’t realise he wants her for her breeding potential alone. He knows that saying this isn’t going to get him far, so he keeps quiet about that and reassures the woman he’s in it for the long term. They discuss marriage - ‘no need’, ‘old fashioned’, ‘outdated concept’, ‘patriarchal nonsense’, etc. Or ‘not yet, let’s have kids / move in together / wait until we’re more settled / save up for the big wedding’ etc. The man has little to no intention of marrying, but the woman believes he does want to at some later point.
Kids arrive. They’re still not married. No need now, right? The kids are already here and they’re still together so why get a piece of paper to prove things are stable?
Only one person is committed to this relationship and it isn’t the person who knows where it really stands.
Eventually the man moves on, finds someone else and quite often marries her.
There are a lot of single unmarried mothers out there. Not all of them by choice. They’re the ones who take to MN distraught that they’ve been left holding the baby. If they knew in advance that their partner just wanted to procreate and that they came along at the right time, how many do you think would have willingly tried to cultivate an emotionally intimate relationship with their former dp?
They’d have saved themselves the heartache and registered for receipt of donor sperm. It’s basically what they ended up with anyway.