I think the dating metaphor that's been mentioned is apt though. I'd think that even if you hadn't started with the "pregnant" metaphor. Getting someone excited about an idea is always a gradual process, not just when trying to find a cofounder. It's the same when you're trying to sell a product, too.

You need to gradually woo someone. Present the rough idea initially, but then allow them to start taking ownership of the idea themselves by brainstorming with you, and start helping you to shape it. Cofounders (and most early employees) need to feel like they're working on "their" idea, not someone else's. And chances are, your idea still needs a ton of refinement yet, so you actually really need the help.