it is allowed for a muslim man to enter his wife in her vagina from any direction he wishes - from behind or from the front. about this allaah revealed the following verse:

there are also various hadith on this subject, of which i will give only 2:

1. on the authority of jaabir who said: "the jews used to say that if a man entered his wife in the vagina but from behind, their child would be cross-eyed! then allaah revealed the verse: "your wives are as a tilth unto you; so approach your tilth when or how ye will;" [al-baqarah 2:223]. the prophet said : "from the front or the back, as long as it is in the vagina". [al-bukharee and muslim]
2. on the authority of ibn 'abbaas who said: "the ansaar, who had been polytheists, lived with the jews, who were people of the book. the former viewed the latter as being superior to them in knowledge, and used to follow their example in many things. the people of the book would only make love to their wives from the side, this being the most modest way for the woman, and the ansaar had followed their example in that. these people from the quraish, on the other hand, used to expose their women in an uncommonly manner. they took pleasure in them from the front, from the back, or laid out flat. when the makkans came to al-madeenah at the time of the hijrah, one of them married a woman from among the ansaar, and began doing that with her. she disapproved of it and told him: "we used only to be approached from the side, so do that or stay away from me!" this dispute became very serious until it reached the ears of the prophet. so allaah, revealed the verse: "your wives are as a tilth unto you, so approach your tilth when or how ye will;" [al-baqarah 2:223] (ie. from the front, the back, or laid out flat). what is meant here is the entry which produces children." [aboo dawood, al-haakim and others: hasan isnaad and is supported].