The problem is that many women are curious or fantasize about anal sex, but they are too afraid to try it because of possible discomfort, safety concerns or bad previous experiences.
She might have had an ignorant partner before.
Maybe she had anal and it was one of her worst, most painful sexual experiences ever.
Instead of asking directly, a good approach is to make her want to try anal sex by herself.
From a scientific point of view, anal sex is pleasurable for women because of concentrated nerve endings around the anus and along the rectum walls.
Use this information to your advantage in arousing your woman.
The best way to influence your girl to try anal is by introducing her to pleasure through anal play.
Getting her accustomed to anal play will gradually let her relax and make her more open to the idea of anal sex.
Every time, she'll allow you to 'play her ass' a bit more, finally getting to full anal penetration when the time is right and both of you are prepared.
The key to this approach is to associate anal stimulation with sexual pleasure.
The best time to do so is when you are eating her out or in the middle of the intercourse, when she is turned on the most.
Squeeze her butt cheeks and wander with your fingers around her crack.
While going down on her, try pushing her knees upwards, toward her face and run your tongue as close to her anus as it's comfortable for the both of you.
Keep an eye on her behavior; if she looks as though what you're doing is arousing her, keep on doing it.
On the other hand, if she starts backing off, slow down a bit.
Try next time and she should be more comfortable.
Repeat this over and over until gentle rimming becomes a regular part of foreplay/sex.
If you're getting positive feedback from your girl, the next time you go down on her, lube up and gently brush your finger over her anus, circle around it and look for a response.
If she responds positively, continue.
In case she backs off, get back to arousing her by other means.
Act like nothing happened.
Continue to massage her crack while giving her head.
By the time you start playing with your finger she should be dripping wet and much more open to trying 'dirty stuff'.
It's important that you don't penetrate her until you get some more positive feedback.
Assuming that you were successful, now it's time to lube your finger up some more.
Circle around and try penetrating her ass a bit.
Don't move your finger too much at first.
Just hold it in place for a few seconds, so she gets used to the feeling of something inside her backdoor.
If she likes what you're doing (and she should by now), penetrate her with your whole finger and start making gentle circular motions with your whole wrist.
She will feel this motion in her vagina.
It's been proven that simultaneous stimulation of a woman's anus and clitoris or nipples is more satisfying than plain anal stimulation.
Lick her breasts, vagina and kiss her whole body while fingering her ass.
Getting your woman aroused goes a long way toward reducing inhibitions.
Take advantage of her uninhibited state and you can progress to the point where she'll allow you to anything to her.
'Anything' in this case means penetrating her anus with a finger, then two, or maybe even proceeding to using a small sex toy.
When you get to the point that she likes and is turned on by anal play, you are just a step from anal sex.
Take your time, and be patient.
After a while, advancing from anal play to anal sex will be a logical 'next step'.
Try the tactics above and tell me how they worked for you.
I love to hear a good success story.