This isn't really a cheat, but it's the easy way around. to get two sims to love each other right away, make two sims of the same astrolic sign (like cancer and cancer) compliment each other and do other stuff until they want to kiss. then have them kiss ten times or so to make them love each other, and presto love on the first day.
More tricks, tips and cheats for this game are right here - The Sims cheats
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