If God was capable of creating human beings to begin with, without benefit of pre-existing males or females, then it's no great leap to suppose he could create a human being with the "assistance" of a human female.
Presumably when he created Adam he created an entire body. To effect a virgin birth he would only have to create one cell, i.e. a sperm cell. Or less: he could have caused an egg in Mary to split in a process akin to parthenogenesis. He would have had to create a Y chromosome, and he may have decided to create other genetic material.
Of course, whether you believe in a God capable of such creation is a pretty fundamental question.
BTW, anti-Christians often say that people in Bible times believed in miracles because they didn't understand science and so didn't know that these events were impossible. This is ridiculous. The authors of the Bible were well aware that the virgin birth (and other such miracles) was impossible by every known law of science. That's why they described it as a great miracle, rather than a curious side note. If people back then didn't know that it was impossible for water to turn into wine, they wouldn't have said that Jesus performed a miracle, they would have said, "How convenient! The water turned into wine just when we needed it."
Reply to Mr Gravell: Do I really need to give examples of people saying that the folks in Bible times only believed in miracles because they were ignorant of science? Okay, in a quick Internet search I don't find those exact words, but here are some examples of essentially that idea:
In those parts of the world where learning and science have prevailed, miracles have ceased; but in those parts of it as are barbarous and ignorant, miracles are still in vogue. -- Ethan Allen (quoted on Wikipedia "Miracle" page)
The people who tell these stories about miracles would make good storywriters. They would not make good science fiction writers. They possess little knowledge of science. --
http://www.jovialatheist.com/miracles.html (In context, he's talking about the writers of the Bible as well as people of later times who believe them.)
We may, in fact, say that a miracle is an event of which the causes cannot be explained by the natural reason through a reference to ascertained workings of nature; but since miracles were wrought according to the understanding of the masses, who are wholly ignorant of the workings of nature, it is certain that the ancients took for a miracle whatever they could not explain by the method adopted by the unlearned in such cases. -- Spinoza. http://www.sacred-texts.com/phi/spinoza/treat/tpt10.htm
Skepticism wasn't what it is today. Someone who tells you today that a demon jumped out of a tree, or a giant sea monster devoured a ship is likely to be ridiculed or sent to the Daily Sun. However, such tales were quite common and widely accepted with little question in Christ's day. -- http://www.infidels.org/library/modern/kyle_gerkin/objections_sustained/obj2.html
But a miracle means a violation of a natural law, and there can be no proof imagined that could be sufficient to show the violation of a natural law; even though proof seemed to show violation, it would only show that we were not acquainted with all natural laws. ...
Primitive and even civilized people have grown so accustomed to believing in miracles that they often attribute the simplest manifestations of nature to agencies of which they know nothing. -- Clarence Darrow. http://www.infidels.org/library/historical/clarence_darrow/why_i_am_an_agnostic.html
In ancient times it was natural to ascribe the violent acts of nature to a pantheon of mischievous or malevolent deities. Calamities were often taken as a sign that we had somehow offended the gods… Ignorance of nature’s ways led people in ancient times to invent gods to lord it over every aspect of human life.” – Stephen Hawking and Leonard Mlodinow, The Grand Design, Bantam Books, p25