People go on dates for any number of reasons beyond potential life-partners.
A relationship is starting to get serious when you and your partner proudly display each other as significant others in the public eye.
You have no problem mentioning each other in tweets, posting photos together in Instagram pictures or including each other in Facebook status updates.
You've reached solid ground, and while many things in the world remain gray and uncertain, you generally know where you and your partner stand. There are some blatant signs that a relationship is getting serious: You might have a discussion about making the relationship exclusive, discuss moving in with each other and of course, there's no greater sign than when a ring makes an appearance.
Aside from the formal discussions, game-changing decisions and Kodak moments, there are some subtler ways of knowing if things are going from gray to golden. This goes beyond making out or the occasional butt pinch in public.
Of my friends who entered into relationships in the past year, every single one of them first entered a period of exclusivity before even remotely venturing into "boyfriend/girlfriend" territory.