Nope, it is impossible to get the scene without multiplayer. You can get assets up to around 7,000, but EMS will max out at half that total without multi.
And the magic number is 4,000 EMS to see the scene you are referring to. According to internet sources however, you will only see that scene depending on conversation choices made preceding the ending.
IIRC, the official guide says that if you manage to
keep Anderson alive (which means successful persuasion of TIM to shoot himself in the head), you only need 4000 EMS to see "Shepard is alive" ending. But if Anderson is dead in your game, you better have 5000 EMS if you still want to see "Shepard is alive" ending.
At what point do you have to decide who you want to bang?
The sooner the better. With some couples, it's implied that they already have sex before the major sex scene even take place.