Speaking to Reuters, the actress said that the film's intimate acts are in good taste and have never been shown on screen before.
"I thought, 'I've never seen that before in a movie and I'm proud to be the first. Good on us for making that happen'," she explained.
"As the woman in the situation that was in question, I found absolutely nothing scandalous, tawdry or disgusting about it."
Williams went on to say that she was "shocked" to hear of the film's initial NC-17 rating, adding: "It shocked me. It felt like a slap on the hand when I found out about it.
"When we shot that scene, both Ryan and [director] Derek [Cianfrance] said to me, 'If this bothers you when you see it in the movie, we'll take it out'."
The MPAA later reversed Blue Valentine's rating to an R, which is suitable for all adults over 18.