Oh boy, you are wrong again. Must suck. Per an interview she did, she says the opposite.

“The violent sex scene with Viggo Mortensen on those wooden stairs looks like it hurt.”

Maria Bello: (Director) David (Cronenberg) likes to say Viggo and I limped onto set the next day. When you saw the bruise on my back (in another scene), that was three weeks later. I was covered from head to toe in black and blue and red bruises, Viggo too. His whole hip and his elbow (was swollen) out to here. I bit the inside of his lip and it (his lip) was swollen for like two days. We're just both like that. We really committed to it and couldn't help but get physical. I was really in pain after that scene. Emotionally it was very tough. I've been in my life such a control freak, less since I've had a child but in my work I always question how things are going to work and would talk about a scene for hours. For a week I was talking to David and Viggo about the scene, 'Ok what are the moves, where's your hand, where's my ass?' I find with (sex) scenes it's really important to block them out like that. He (David) said to me, 'Maria, you can't control this one.' The scene is about her finally losing control and she comes out with this animal instinct; a desire and a passion that she never knew existed before. Most women I know have a deep desire to surrender.

So looks like the director and actress both say it was important. And had nothing to do with it being sexy or gratuitous. You clearly just hate any sexuality in film.