I witnessed this 'conversation' today and I didn't feel like I could say anything without making the situation worse.
Working with two colleagues today (we are NHS and usually share an office). One is married with two DCs and one on the way. The other is single. Married colleague decided to start questioning single colleague about her intent to have children and whether she wanted to meet someone to settle down with. Single colleague was clearly uncomfortable with the questioning (it was repeated several times) and tried to deflect it by saying she didn't have firm plans etc. Married colleague just wouldn't let it go. She was sort of playing it like she was joking around but it was just super uncomfortable to witness, so I can only imagine how single colleague felt. She then started questioning her about other aspects of her life, again repeatedly.
Aibu to think this is not a subject that you should really pursue at all? Single colleague may have issues having children or things going on in her private life that she didn't wish to share. I'm feeling terrible that I didn't intervene but I really honestly don't know what I couldn't said that wouldn't make single colleague even more uncomfortable. If I'd jumped in and said the questioning was inappropriate, I suspect married colleague wouldn't then asked single colleague if she minded.