What's the best way to do this?

I have seen scripts that have just written that the two make love.  I have seen scripts go into detail like "Basic Instinct."  

How do I write it without it coming across as soft porn?  Any suggestions?

tweak

Is the actual lovemaking act itself (positions, moans, facial expressions) relevant to the story? Or are you just showing two people having sex? If it's the latter then just say they're making love (we all know what goes where and what it sounds like, so no need to state the obvious).

I agree with Sniper.

Sex scenes should be treated as scenes, with a protagonist and antagonist, with goals and conflict. If you just want them to have sex, but showing the sex isn't the purpose, then hint at it. Show two naked bodies, and a door closing on the camera.

FRACTURE, the script, opens with two characters just finishing. Look it up.

It depends on what your goal is for the scene, and the characters.

Every scene needs to carry weight. It needs conflict and it needs to move the story forward towards the next scene. Scenes either start positive and end negative, or start negative and end positive.

As suggested below, if this scene is about bringing two people together for the benefit of the audience, then I would make it no more that a couple of lines. Hint at it. If there is something within the act of making love, then you need to show that.