Well, the decision to only have one child may be determined by fertility. If I wasn't as old and decrepit, I would happily leave it for another few years because I'd be happy to devote myself to DD right now and can't imagine 'sharing' myself. But, realistically, we have to try now or it will be too late.

See, the thing is, things just change. Don't worry too much about the future, things will work themselves out regardless of whether you want to stick to one or you want to go for another.