This can be done with any couple, it will be easier if you try this with a couple that are already BFF or going steady (going out).
I'm not sure if this can be achieved with a same sex couple or not. EDIT-
takuto neko has confirmed that this is possible with a same sex couple. Thank you for letting us know.
I started my game with one guy Ilo Skully (available to download off the exchange), and he wanted to become a forensics something or other in the police. So the first thing I do is get a job in the police force. My partner is called Penny. I start to chat with penny to try get the "Working friends" achievement. After I became best friends with my partner and boss. I made the move to start flirting with my partner Penny after a bit of flirting, compliment appearance ect. more options will appear like ask to go steady, jump into arms massage. ask to go steady if you can then keep massaging, jumping it arms, until whisper in ear appears do that then woohoo should be an option under romantic choose this option (at home) and they will kiss and go off to bed. they will play around above the sheets for a bit then disappear underneath.... achievement unlocked.
I think I managed to get this achievement before "First kiss".
If you don't want to be in a relationship, select be mean and break up... or try this solution without asking to go steady just be extra nice.
Also worth mentioning if they are not enjoying you making a move try and be friendly for a while before you start using the romantic options.
Your sim must be happy for this to work.
Be patient if your sim is in a bad mood this will be frustrating, plus it can take a good few in game hours to do.
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