In The Sims 1: You can't.
In The Sims 2: You must either have an elevator (The Sims 2: Open for Business) or photo booth (The Sims 2: Nightlife) at a community lot. Have one of your sims enter either of these and then have their significant other select "WooHoo" or "Try for Baby" after clicking on it. The significant other will enter and they will get a special memory of having Public WooHoo.
In The Sims 3: Have two sims visit either the graveyard, city hall, science lab, or movie theater together. Click on the rabbit hole building there and select "Tour _______ With...." and then the name of your sim's significant other. The two will enter. You must then click on the building again and select "WooHoo" or "Try for Baby". There is a chance that the two sims could get caught and run out in their underwear, too! xD