Slim people hear this in response to eating healthy food, too. In my experience, people are genuinely impressed/inspired to see healthy meals, or feel guilty about consuming junk they brought with them (so their comments are actually to themselves, small reminders that this is perhaps how they ought to be eating).
I’m a woman, and most of the women my age (30s) view pregnancy as the source of a lot of body horror. I know I’d be a lot more interested in having a child if I knew for a fact that after giving birth I would have no chronic conditions. A lot can and does go wrong, even if it doesn’t result in death, and this is more and more common as average child-birthing age goes up.
Women react differently to things like intermittent fasting given the vastly different hormonal profile to men. I’m much like your ex (I can’t think properly until I feel full, which means a significant breakfast) and anecdotally I have noticed that men seem more likely to go for longer fasting periods. Women’s bodies really don’t like to think they’re starving.