Proposing is a lot harder with the Sims 3 than the other games.
You have to get their relationship up at it's highest level and continually do 'pleasant' social options until 'ask to go steady' is one of the options. Just keep flirting, chatting and flattering them. It helps if you have a positive social trait like flirty or good humoured.
Eventually the option will present itself but you have to have them interact for a while before it does