Not necessarily.
Sure, we all know men who’ve said they’re against kids and marriage who go on to have those things with a new partner once their previous partner is too old to have kids.
But I also know men who say this and then remain unmarried and childfree just as they said they would.
It’s not really relevant or useful to anyone though is it? Surely the point is if a man doesn’t want marriage and kids with you, you walk away and find someone who does. Whether he goes on to change his mind with someone else or not isn’t relevant or any of your business once you’re no longer together.
You can’t really blame a guy if he’s open about not wanting kids or marriage and his partner remains with him, at least he’s honest and she’s choosing to stay in the relationship knowing those things aren’t a possibility. It sucks if later on you split anyway and then he changes his mind with someone else but people are entitled to do that, of either sex.
I do think eighteen months is pretty fast to be getting married but at their ages I’d say by eighteen months you should both have a clear idea of what the future holds in the relationship and be on the same page about marriage, kids and general timelines. At eighteen months we knew we both wanted kids within a couple of years and that we wanted to be married before we had a child (but we were both happy to start trying before marrying and marry during pregnancy as we didn’t want to risk waiting for fertility reasons). Moved in to a rental at one year, bought a house at 2.5 years, got pregnant at 2.5ish years, got engaged at three years and are having a ten week engagement so we’ll be married before the baby arrives. None of that was a huge surprise to either of us, it’s normal to have conversations from the beginning around what you want from your future.