An odd guestion

Hiya,

I've got a question about a phrase which might sounds a bit odd, but I do hope it is appropriate to address it here as I was wondering what meaning it conveys.
So here's the phrase : ''I like your back'' and I know it should be clear from the context, but it really confuses me. Just to add some more information to the context, it's about two people in a bedroom having a good time and one of them tells the other ''I like your back''.
Since I am a non-English speaker I asked a British-born friend and she was a little confused as well. So, here I am trying to shed light on this simple phrase.

Thanks!
M.
Bulgaria