How do the girls feel if "IT" is inserted from BEHIND?

I want to have with my girlfriend in her "BACK" hole. But she is very reluctant for that. Says she won't allow me. And because of this our relationship almost got to the brink of break up. She said that its painful and gives no pleasure, but i believe that its too tight, and would surely give pleasure to her and me as well. Please tell me how does it feel if i insert "IT" in her back rather in her ? Is this because of the pain she's too reluctant? Will it be really painful for her? If yes, plz tell me and i won't do it, if NO, then why does she refuse? Does it not give more pleasure than ? I think it would give both of us more pleasurable sensations than the FRONT way. Plz tell me so that i don't harm her or our relation. Or, plz tell me how can i persuade her to doing it in her BACK? Plz help me.