How do you write an intimate scene (NSFW)

I’m writing the script for my graphic novel and have come to an impasse. There’s about 3 planned sex scenes (listed in approximate order of appearance):
Between Shift and Jeremiah two superheroes who meet on the battlefield and find an animal attraction and have a loose, NSA relationship
Between Ruth and her Professor in a flash-back to college, we get a window into the main character’s past, where she becomes attracted to (and possibly seduced by) her professor, a married man. Ruth ends up falling in love with him, after which he leaves her and works things out with his wife.
Between Ruth and Anthony Possibly the messiest of affairs, my main character and the villain of the story fall in love and make sweet, passionate love (remember the scene in Terminator?) just after he “saved her” from Shift, superheroine and Ruth’s good friend.

I’m not looking for someone to write the scenes for me, but how does one go about writing the build-up and sexual tension in a comic or screenplay or other such medium? Does anyone have any ideas or jump-off points? Is there a comic I should read or movie I should watch for ideas?