Finding a few contacts and seeing how those Sims get along naturally is a good idea if you're going to play them in a family together. As the Sims use Romantic Socials on one another, the relationship meter will rise or fall based on whether that interaction was received favorably or not.
At different combinations of friendship/romance levels, you will get a different title for your Sim's relationship with that Sim, from Total Opposites to Soulmates.
As the level of the Conversation rises through Suggestive Encounter, to Amorous and Steamy Exchange, more Socials become available.
Still, a couple should not have to expect to build up with their boyfriend or girlfriend before going in for a kiss or even woohooing.
Know that Romance is not restricted to Adults in The Sims 4.