They have to at least be romantic interests - in addition they have to think that the other person is "extremely irresistible" (comes after flirty and then alluring). After that 你 can select woo hoo 或者 try for baby.
If the couple is married 或者 engaged (probably even if they are boyfriend/girlfriend) 你 can bypass all that and make both sims "relax" on the same bed. Then 你 can just make them cuddle and then woo hoo 或者 try for baby (it's usually best to make them 吻乐队（Kiss） and then make out first to get them in the mood).
They're also 更多 likely to conceive and just do it in general if they are both really happy and not in need of any essentials (hunger, tired, need to use the bathroom). Good luck!
