Wow, I hate give Islam credit for anything - but I certainly couldn't disagree with this edict!
I am sure your wife smiles... LOL
She CAN refuse.....but if she's smart, she won't.
The Quran in Sura (Chapter) 2:223 says:
Your women are your fields, so go into your fields whichever way you like . . . . (MAS Abdel Haleem, The Qur’an, Oxford UP, 2004)
(This is actually a response to a question about allowable sexual positions. The questioner starts off by saying, "The Jews say that if you enter a woman from the back the child will be cross-eyed.") - I'm not kidding!
The Prophet said, "If a man invites his wife to sleep with him and she refuses to come to him, then the angels send their curses on her till morning." (Bukhari; see also these two parallel hadiths: Muslim nos. 3367 and 3368)
There are a ton of translations out there of the Koran and the Hadiths which are commentaries on the Koran. The Hadiths are collections of sayings attributed to Mohammed and commentary.