The Sims 2 Guide and FAQ

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5.3 Love Between Two, Three or More Sims
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As has been mentioned a number of time you Sim can fall in love with any other
Sim, regardless of Sex, but it is yourself that controls this. There are ways 
to do this. Not only that, if you make your Sim more sexual towards a gender, 
they will remember that, and take action in that direction by themself.

Once you have made a Sim fall in love with you, and have fallen in love with 
that Sim, the relationship changes in many ways, and the way you need to treat 
that Sim changes.

   Note - If you don't want your Sim to fall in love with another Sim, stay 
          away from the Kiss, Hug and Flirt interactions.

If your Sim is a Romance Sim then many of the Wants that you will have to 
fulfil will involve having relationships with multiple Sims. This is a lot
harder to do than in the original Sims. Knowing how to do it will help through,
all you need to do is understand how not to make the other Sims Jealous.

A Sim will only get jealous if, A. There is a romantic interaction between a 
Sim they have a crush on or are in love with, and they are in the same room as 
when the interaction  takes place. B. If they are on the lot if WooHoo takes
place.

Taking this into consideration it is very possible to keep on as many romantic
relationships as you want as long as none of them live with you. Then again, it
is possible for you to have 3 or 4 lovers live with you.

   Note - It is important to remember that just because the "other" Sim is not
          there, it does not stop your Sim from getting caught. Your Sim's 
          cheating can be witnessed, if it is witnessed, that witness can chat
          with other Sims about it.