Another quickie recommendation would be to read the chapter in Sol Stein's "Stein on Writing" book about writing love/sex scenes. He's in the camp of the "fade to black" populace.
If you are uncomfortable about writing the scene, boil it down. What is the most important thing you or the reader need to take away from the scene? Is it that they get together? Or is it that they have gotten together? Is it something that happens while they are together? What brought them to this point? What will be the fallout? What things leading up to this are the most important? What superfluous details are you adding now that don't relate to any of the above questions? If there are such details, can the scene live without them?
The real reason your dialog is coming out stilted is because subconsciously, or consciously, you are trying too hard to make it sound like a sex scene and forgetting the focus of your scene or scene question/scenario. Ask yourself first, why are you putting it in? Is it because there are so many books out there now skirting the fringes of public decency in order to sell better? Is it because you were asked to put it in by an editor/agent? Or...is it because it is necessary to the plot? If it is one of the last two options, you will need to break down some of your inner restraints and let loose. If it is the first, don't put it in. Period.
Amy