There are many things that can put a woman off having anal sex with their partner. Firstly- it can hurt- unless you are very gentle- there is the possibility that it might make her bleed- so this anticipation of discomfort is incentive enough not to want do it. Even if you are gentle upon entry- there is still the potential to cause damage to your partner's anus if you get carried away and are a bit rough with her during.

There could be mess- if your girlfriend doesn't empty her bowels at least an hour before or doesn't take an anal douche- there could be an accident. She may be embarrassed about this happening which is putting her off too.

If you have anal sex, then pull out and have vaginal sex- it can transfer germs from the bowel into her vagina with the possibility of causing infection.

If she doesn't want to have anal sex with you and it's so important to you- then perhaps you need a new sexual partner. There are many other things you can be doing in place of this to have a fulfilling sex life- it all depends what means more to you- anal sex or your relationship.