For mysterious reason, the men become unable to transmit their Y chromosome, which mean that from a certain point, only baby girls are birthed. The number of male slowly decrease and despite the heroic effort of the last remaining potent men, the demographic start collapsing.
To survive, the humanity have to find a way to reproduce without male in short delays. Cloning is considered but there is a major problem. It's horribly expensive if you want good result. Reproducing only through cloning would ask way to much effort to the society to be sustainable.
Gladly, some scientist discover a new drug that change the way a woman ovum are working. Those altered eggs are able to fuse with an other ovum to become a baby. Yes! The human kind is saved.
But wait, ovum are still unable to move by themselves, are they? So even with this drug, you still need a complex and intrusive medical procedure to get a woman pregnant. We just don't have enough doctor to stabilize demography this way.

People imagine many solution to make the procedure more simple but fail one after the other and the number of human on the earth keep getting lower. This despite the frantic efforts of the few remaining males, disregarding their own safety and not counting hours to slow down the decline of the hommo sapiens.
But finally, someone come up with the Solution. In front of the congress, a young woman proudly explain to her audience how she managed to impregnate her girlfriend without needing any tool or external help except the ovum modifying drug. By her side, stand the living proof of what she is saying: a second young woman looking quite intimidated and heavily pregnant.
How was it done? Well quite simply. The two girl are into hardcore sex since a certain time and love stretching their pussy with unreasonably big object.
The atmosphere become more heavy with each of her world but the future “father” keep on her tale.
She have been a bit to greedy once or twice in her “games” and caused her reproductive system to fully prolapse out of her. And from this point, impregnating her love was straight-forward! She just inserted the end of her pink socks in the cervix of her loved one and milked her own inside-out reproductive system to get her ovums in her love.
Easy as pie with the adequate training.
Except the girl you just finished her speech, everyone in the room look mortified.

… Years latter …

Following the rule of natural selection, now remain on hearth only women that either enjoy or tolerate having their cunt destroyed by obscenely large toy or women that are rich enough to pay for an ART. The others didn't had any descendants.
As it's preferable to start the training early, passed a certain age, breaking it's own cunt is part of the PE activities (and it's graded so don't take it lightly). As maintaining one reproducing system in a “functional” shape require constant care, many thing that would have been seen as sexual nowadays are now mundane. Woman are given themselves a good fisting in public place as naturally as they would fix their hair.
But in this time there are women that want to leave their pussy alone and focus on some other holes. They enjoy more having their anus gaping than their pussy or prefer extract their colon out of their body instead of their uterus. And some can only come with their pee-hole stretched to the limit. Those kind of tastes are seen exactly like homosexuality today.

And the world keep going. But one day come to life a strange baby: instead of a slit, between “her” leg “she” have a strange appendage that now one living have seen anywhere...