Two sims who have a romantic relationship high enough to WooHoo
Buy a Rocketship from the buy mode for 5000 simoleons and start building it. It will take several sim hours but you can have multiple sims working on it. Once built, have both of the sims on the plot and have one click on the ship. Select the WooHoo option and watch the sims get in and blast off. The achievement should pop a few seconds after they land and get out.
If for whatever reason it doesn't pop, just keep trying and check the sims moodlet. If their embarrassed, then it is possible that it wasn't a successful woohoo. Also the option "try for baby" does not seem to work for this achievement.
Edit: Fixed a typo. The Cheapest Rocket in the game cost 5000 simoleons not 1000. Thanks to the people who let me know of the mistake.