I think a good build up does a lot of the leg work when it comes to sex scenes in comics/books, just like it does in real life. It all depends what you want to get out of it. Is the scene supposed to turn the reader on? Turn them off? Demonstrate romance? Demonstrate impulsive behaviour?
If it's meant to be romantic and sexy prefacing the scene with tension will get everyone on board mentally, then when it comes down to the nitty gritty you won't need to show much of anything because people's imaginations will already be at work. Characters touching here and there, meaningful looks, sweet nothings, they all do the trick, and with enough of them used in the right way closing the scene with a shut door would be all you need to do to let them know what's happening on the other side.
Alternatively, if the sex scene is being used to demonstrate a different feeling (lonliness, desperation, whatever) cut the build up and dive straight in. Focus on the immediate before and after rather than a build up. A woman walking through a door, taking some money, a glass of water vibrating on a bedside table, and her leaving again are all you'd need.
In either case you don't need to show much of anything to get the point across, so if that's what you're worried about maybe consider how you can set up or close the sex scene to best facilitate your chosen feeling.