WooHoo

$$ WooHoo is used in The Sims 2 and The Sims 3 as an euphemism for sexual intercourse. The game displays a comic animation involving fireworks and an earthquake.

WooHoo interactions can be carried out between two Sims, regardless of gender, that have a very high relationship score. In The Sims 3, Sims don't need to have such a high relationship score in order to be able to WooHoo. During conversation, the other Sim must find them "Extremely Irresistible" in order for the options of WooHoo and/or Try for Baby to appear.

Sims remember all the Sims they WooHoo with, but it's also exceptionally likely to lead to jealousy in relationships.

Play in Bed was present in The Sims, as a predecessor to WooHoo.

Where Sims Woohoo
$$ The WooHoo interaction is possible in several locations:

The Sims 2

 * In bed
 * In a hot tub
 * In a changing booth (Public WooHoo)

Nightlife

 * In a Car
 * In a photo booth (Public WooHoo)

Open For Business

 * In an elevator

Bon Voyage

 * In a tent
 * In a sauna
 * In a hammock

Apartment Life

 * In a helicopter
 * In a closet

The Sims 3
In The Sims 3, there has been accounts of "Friends with Benefits", where two Sims could woohoo without become a love interest.
 * In bed
 * In Military base (must Tour Base first)
 * In Theater (Must Tour Theater first)
 * In Science lab (must Tour Lab first)
 * In City Hall (must Tour City Hall first)

Try for baby
A male and female Sim can alternatively Try for baby, if the female Sim is on her home lot. In The Sims 3, the female Sim does not need to be on her home lot to conceive a baby. Only controllable female Sims can Try for baby on a community lot. Trying for baby carries a chance of conception depending on where it was done. In The Sims 2, Try for baby is not possible for a young adult or an adult female who is within 5 days of becoming an elder. Young Adults can Try for baby in The Sims 3 and Young Adult females are more fertile than Adult females; i.e. there is a greater chance for twins and triplets to occur. If the lot already has 8 controllable Sims on it, the "Try for baby" option will not appear. Zombies and Servos do not have the Try for baby interaction. Only heterosexual couples can try for a baby. Only adult females can get pregnant using this interaction. A male between the ages of Young Adult and Elder can impregnate. Male Sims can also be impregnated by Aliens.
 * 60% if in a bed
 * 25% if in a hot tub
 * 50% if in a clothing booth

In The Sims 3, your chances of having twins/triplets can be increased by purchasing the fertility reward by gaining enough lifetime happiness. Making your pregnant Sim watch kids TV (up to 2 1/2 hours) can also increase chances. To increase the probability of a boy you can eat up to 3 apples, same for girls but with watermelons (must be raw if you have the most recent patch).

Gender and Age compatibility
The following table shows the compatibility of various age groups and genders for WooHoo/baby interactions. Note that, when referring to trying for a baby, the table only states the possibilities for the interaction.

As with all romantic interactions, blood relatives are automatically prevented from WooHooing. Furthermore, because of its wide-ranging implications, WooHoo is never autonomous.

Pets
Pets can WooHoo in order to breed (producing a litter of up to 4 puppies or kittens), but they require a Sim that has a good relationship with at least one of the pets to direct them, and obviously the pets must have a good relationship. The gestation period is approx. 72 game hours, just like Sims.