Some girls actually like it and request it. Its far scientific, but in my personal experience I've noticed a correlation to how pretty a girl is and their interest in anal sex, in that the prettier the girl, the higher the likelihood they may have an interest in it.
This may be for a variety of reasons. Perhaps unusually pretty/feminine girls are just into kinkier things due the degree of attention they've gotten over their lives. But more likely it has to due with the associated higher degrees of sexual dimorphism.
In my unscientific study of this through my personal experiences, i have noticed that the only women who have ever expressed sincere interest in anal sex (and have proposed it themselves) had significant longer ring fingers than index fingers on the right hand. This ratio serves as a marker of ones prenatal exposure to estrogen:
In any event the only women that have ever asked for it had the finger ratio pattern shown in image B above. And all were rather extreme examples of finger pattern B. That said, not all women with pattern B have proposed it, but those with that pattern have been the only ones interested enough to request it.
Personally I don't have any special interest in anal sex. I'm a rather girthy guy, so it can be quite a struggle to get it in there and requires pre-dilation with butt plugs and so forth for it to even work. I mean its fine if she is into it and she is interested in that sort of thing. But otherwise its not worth it. It can be done without those things but it has to be a very slow process initially.
I have noticed however that almost without exception, when done correctly, all women get very wet during anal sex. It might be due to psychological factors but I think that part of it is that the act of anal sex likely expresses fluid from the bartholin glands since anal sex would put direct pressure on the anterior of the bartholin gland, causing increased wetness.
Next time you want to convince a girl to have anal sex maybe say, "but, baby, with anal sex, I will able to apply pressure to the anterior of the bartholin gland, resulting in increased wetness"...maybe all that gland talk will get her going