As qn3t said, the first purpose in those countries is to keep the hymen intact for marriage. I heard many testimonies from Northern African muslim girls who did it.
This is much smarter, easier and cleaner than sewing back the hymen!!!
Now, as you may have seen in porn movies even from akiba-online, anal sex has really, really, nothing to deal with homosexuality. It just another very interesting hole where to perform sex as are her mouth and vagina. While you may as well get STD with anal sex, it is also a good contraception method and I guess this is why religion may forbid it...
However, if you really are a pervert, as you claim, you should have already tried it with your girlfriend. It gives a very nice and different feeling as the shape of her anus is smoother and tighter than her vagina. I actually recommend it to spice your sexual life with your girl. So I will give you my guidance on the matter:
As a delicacy, sodomy requires precautions:
1) Take it easy and slowly with your girlfriend: she must feel safe and as painless as possible, otherwise she may not want to repeat the experience. Clitoris masturbation is always a good way to relax her before and during action.
2) It is necessary to check that there is no feces by inserting one finger. Ideally, the girl should shower her anus beforehand.
3) Unlike vagina, anus does not produce lubricant, so you MUST use a lubricant like vaseline or water base lotions. In very last resort, you can use some food oils like olive oil or butter but bed sheets will get quickly dirty ...
Preliminary:
4) The anus of your girl is tighter and less flexible than her vagina so you have to prepare it to not hurt her. Start gently to stimulate her anus, eventually with your tongue, then masturbate her anus slowly with your lubricated finger, then 2 and finally 3 fingers so that her anus will get expanded and flexible. Always take your time.
5) While fingering her anus gently, you may at the same time, masturbate her clitoris with your tongue, another finger or ideally a clitoris vibrator. Clitoris vibrators generally have a small oval shape (between 3 and 5cm) and a wire linked to the control. You may have seen them in Japanese porn movies. Those items are super efficient and guarantee to give the girl strong clitoris orgasms.
Action:
6) Finally, insert your lubricated penis very slowly and once fully inserted just keep around 30 seconds without moving so that the anus adapts to your shape.
7) Then you can start the movements going slowly and later eventually faster. Slowly, you will fill the deliciously smooth and thin shape of you girl.
8) In order to share the pleasure (it can be painful for the girl particularly at the beginning), I suggest that she or you stimulate her clitoris depending on the position with a finger or ideally with a clitoris vibrator.
9) For the position, you can be in front or behind her. In front, she may feel more comfortable and you can easily masturbate her clitoris, while in the back position, you can go deeper in her anus and it may give you more pleasure. So I may recommend the front position at the first time to get your girl comfortable.
If you follow my advices, I can promise you will not be disappointed... enjoy!