Instructions
Click on the avatar of your male Sim to the left of the screen (the small picture of him). Click on the actual female Sim to open up the action options available. You can make your male Sim complete many different romantic actions, depending on his relationship with her.
Choose simple romantic actions such as flirting and complimenting the female Sim. These are the first steps in working to build a solid relationship. As in real life, moving too fast can lead to some failed romantic endeavors between your Sims.
Click on the tab to the far right of your screen that shows two people standing side-by-side. This tab has the current emotional climate of your Sim concerning the relationship. Click on both of the Sim avatars to monitor their process.
Command the male Sim to propose to his girlfriend when both Sims find each other irresistible (it should say so on the couple's tab to the far right of the screen). Once they reach the level of "irresistible" to each other, it is most likely that the female Sim will accept the proposal.
Click on your male Sim's avatar and then click on the female Sim. When your male Sim finds the female irresistible, you will see a proposal option appear on the action menu when you click on the female. Select it and the male Sim will kneel and propose.