I'm going to share two stories of two very different women, who did not give ultimatums.
I met Jenny when we were 14. Even at this age, as a young feminist and future career woman, she didn't want children. Everyone told her she'd change her mind. At 16, she didn't want children. At 20, she didn't want children. At 22, she fell in love with a man 10 years her senior, Dave, and told him clearly that she didn't want children. At some point, however, Dave decided that he did want children and that Jenny would eventually change her mind. Jenny entered academia, where she grew into an even more liberal feminist and unsurprisingly, did not want children, a life choice that was very much supported by her environment. Ultimately, she told Dave that under no circumstances was she ever willing to have children. He walked and she let him. She never changed her mind and, last I heard, had met a man with similar goals and was pursuing her PhD.
I met Sally when I was working at a movie theater in college. She was pretty, slender, smart, and was dating another employee, Cody. Cody was good looking, confident, funny, social, ambitious, and often a complete asshole. They both came from good families and had been dating for nearly a year, at 19. I don't recall if they intentionally followed each other to college or met there, but just from the oversharing young people do, I knew that Cody had been Sally's second partner and she had been his first. Love was in the air and they both assumed that, after college, they would eventually get married.
Although Sally wanted a career, even then, she knew she wanted to be a doting wife and mother. At 19, Cody seemed game for the former, but less interested in the latter. Finally, he told Sally that he didn't want kids... and she immediately ended it with him. She told him she loved him, but that she was not willing to spend her youth on a man who wouldn't eventually want children. She had to know that if they married, children would be in their future. She didn't throw a fit or demand promises. She walked... and he didn't let her. A few months passed and Cody thought it over, consulted friends and family, and decided that he would eventually want children. He genuinely loved Sally... and she did not immediately take him back. Unsurprisingly, she felt that he'd only given in to an ultimatum she didn't intend to give. I don't know what Cody did or said to make her feel differently, but in time, they did get back together. Last I heard, they're happily married and on baby number two.
A tale of two women, the men's decisions are irrelevant. These women, however different, both vetted carefully, at a young age, for a man with common goals. Upon discovering that their choices weren't consistent with where they wanted to go in life, they set aside their feelings and walked. Ultimatums are always a terrible idea and neither of these women gave one. They calmly and rationally stated their deal breakers and took a future they didn't want off the table, accepting the consequences. One found herself single and the other found her man had a change of heart. The difference here is that Jenny had endless years to find a man who didn't want children, especially in her environment, while Sally had only a handful, making it all the more important for Sally to vet with her endgame in mind.
For all the women here who find themselves considering giving an ultimatum or even waiting him out, I'd advise you to rethink. Consider what you want from life and vet accordingly. If he tells you he sees a different future than you do, walk... not so he'll run after you, but to free yourself up for someone with a similar vision. There are many, many wonderful men out there and the earlier you begin vetting well, the better chance you'll have of landing one.