If your Sims has a goal like "Get in a romantic relationship", "get married", and/or " WooHoo." Heres a quick way to do it but only for the guy I haven't tried the girl yet. But theres a character named Nina who lives next door to you, blue house on the left side. Make sure to get to know her first but then you can Flirt, Tell a Flirty Joke, and Compliment Personality a lot. Soon before you know it you can get into a relationship with her. Next with be to get a Date, Partner, engaged, Marriage, and then Spouse. To achieve this , when you start to date use "Hot Smooch" and " Hug" (i forgot the action name) a lot. Then you'll be partners and you can soon propose to her. After that you can WooHoo with her, you can do that when you start to date but I think its more polite to wait, that way you'll stay dating. Hope this helps

~Enjoy.