SK have you ever considered that stimulation of the cervix or thrusting in general can cause pleasurable contractions in the uterus? Similar contractions that occur in other orgasms? The uterus has a role in orgasm. Vaginal stimulation can cause movement and stimulation of the uterus leading to similar sensations as occur in clitoral orgasms except not localised to the clit.

I know this is possible because it happens to me. I even get "womb clench" orgasms when I'm mentally stimulated enough and not even getting any physical stimulation.

The vagina itself may not be as sensitive but the uterus is, and so is the clitoris which can be stimulated from behind as well.