As far as I remember, the Greek for this "arche ktisis" means the beginning of creation. Not beginning as in "the start", but beginning as in "the process / act by which someone begins something", or to be simple "begin-ing". The period of time in which God began creation lasted a few billion years, according to evolutionists, and so there is no problem between an old creation and that verse. Besides, the human species from the time when it was human - whether it evolved into humanity or was directly, supernaturally created - had always been male and female.
It is helpful to note that in a YEC viewpoint man was created on the 6th day, which is not exactly "the beginning" of creation either.
Here's another explanation of it, this time from an OEC viewpoint:
http://www.heritagebbc.com/archive2/0110.html
What does the Scripture mean by the beginning of the creation? Does it mean from Genesis 1:1 or does it mean from the creation of Adam and Eve, i.e the human race? The word creation is ktisis in the Greek, the act of founding, establishing, building, etc. What God is talking about here is the foundation, or establishing of the human race, male and female. The word beginning in both verses comes from the Greek word arche or the beginning, origin, the first person that commences, the first person or thing in a series, the leader... In other words, the first people in the human race, Adam and Eve.
Christ is simply saying that from the beginning of the human race, God did not intend for men and women, once married, to divorce. Then he quotes Genesis 2:24 in Matthew 19:5,
Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
This is not a proof that the earth is only 6,000 years old.The Bible is an inspired record of Gods dealings with man. It begins with man and ends with man. It is not a history of the planet!