I don’t like the word slag. She’s behaved badly but for me the word is used to put down women who enjoy lots of sex which there is nothing wrong with(when nobody is getting hurt).
I think it is less that she felt the biological clock ticking and more that having a baby is often a way to ensure a man is tied to you. She thinks she will be the one to change him and he will stay around for her. It’s why so many women have children with men they know are idiots, they think it will be different with them. Of course they are both responsible for the pregnancy, he should have worn a condom if he didn’t want a child.