Instructions
Choose your mate. Adult Sims can marry any other adult Sim in the game, including NPCs (nonplayable characters, such as the mail carrier). At the onset of the game, other Sims visit your residence to offer their welcomes. You can choose one of these Sims or visit different houses within the neighborhood to meet other Sims.
Begin with conversation and build your friendship. Your options are limited to "Chat," Funny" or "Mean" before you get to know the Sim well. Chat with the Sim to get to know her. Compliment her when the action becomes available. Avoid anything from the "Mean" action group, such as slapping or insulting. This causes the Sim to dislike you, which doesn't improve the odds of a romance.
Choose from the "Romance" or "Flirt" action group. Actions such as "Massage" and "Make Out" and other romantic gestures become available once you and the other Sim know each other well. Continue to talk to and spend time with the Sim. Pay attention to your romance-status bar. The higher it gets, the more options you'll have available.
Choose the "Woo Hoo" option. Once your Sims have a romantic link, the "Woo Hoo" option becomes available. Select the bed and choose "Relax," and then ask the Sim to join you. Once in the bed, select "Woo Hoo." This furthers your relationship with the Sim and raises your romance status. You can also "Woo Hoo" while in the jacuzzi or hot tub.
Select "Propose Marriage" from the action menu. Once your romance link is high, preferably in the 80 to 100 range, the "Propose Marriage" option becomes available. You can also propose "Move-In" first, if you want your Sims to live together before you propose marriage. This, however, is not required to get engaged.
Watch the Sim's reaction. He will either accept or decline your proposal. The onscreen dialog box will let you know. If the Sim accepts your proposal, the two of you are engaged. If he doesn't, continue building your romantic relationship and try again.