My wife LOVES getting fucked from behind.
When we first began dating, she always asked if I wanted to do her from behind, or often would say she wanted me to fuck her from behind.
This usually involved her going ass up so I could pound it.
And she would always ask if I liked that, does it feel good, etc.
She had a NICE big, round ass that looked damn good from that angle, so I never had any problems accommodating her.
The way I figure it, she would do the same when she would take some new or strange cock.