It is not really this easy in real life mind you. Get your Friendship level up to 80 or 90. If the background is very red and blurry, select "Be on" the socialize menu and go to a bed. Select "Relax", then choose the bed again. Select "Woo whoo" once, then press B to get out of the bed. Select "Socialize" and if you are in love, select "Propose" and hope that the other Sim says yes