I remember the first time anyone ever put my legs up in the middle of missionary. He swung both of my legs over his shoulders and repositioned his hands on both sides of my head. When he bent down to kiss me, my first thought was, "Holy shit, I'm more flexible than I thought." My second was about how exposed I was. His face buried in my neck, I knew he wasn't looking, but still—there's something inherently more private about sex when you can count on the protection of your legs.
The first time I tried this position, he hit my cervix. Not in a painful way, as he didn't ram into me, but a strange combination of smooth and full. He slipped in and out easier than he had a mere 10 seconds before, but he was so much deeper. I couldn't wrap my mind around where the space came from—just that we were somehow having sex to a stronger degree than we were before. I could feel everything.
The next time we had sex, I grabbed the spot behind my knees and pulled my legs up to my shoulders myself. He chuckled. "So you liked that, huh?"