Fran hon, I'm in the same boat and I cried reading your post
because I've had the exact same thoughts - what if the person I'm with finds someone who CAN bear their children and that's their excuse for leaving? It's easy for someone young (especially a dude) to say they don't care about kids, but when they get older they might.
I dunno. It's a weird feeling, it's disappointing even though we never really planned on kids. Not quite sure how to handle it so I'm not sure what advice to give you, but hang in there. ***hugs***