I'm not sure its safe to come in between a marital disagreement!  That being said, if you're a soon-to-be or perhaps already pregnant woman, than your man should know that you are right and end the argument for his own good! : )

Dr. Landrum Shettles was a controversial physician who wrote about the method of naturally choosing the sex of your baby as you describe.  The book is called How to Choose the Sex of Your Baby, and although it was originally published in 1970, there are newer versions out there on Amazon, etc.

That being said, I'm not convinced that you should take the word of a single doctor as being truth.  For example, the Shettles method states that the woman should not orgasm if she wants to have a girl.  Apparently having your hubby wear tighty whities is also better for girl baby making.  Is this rooted in science? Doubtful since there aren't examples of corroborating research that are readily available.

Dr. Shettles is known for other work in unethically experimenting with human in vitro fertilization techniques and attempting to race to create the first test tube baby.  Personally and professionally he's been plagued with controversy and mixed opinions.  You can read more about him in this New York Times magazine article:  http://www.nasw.org/users/robinhenig/SecondBest.htm

So - its not crazy that you've heard about this type of sex selection methodology. But the methodology itself is by no means peer reviewed science. 

Your husband is right, the method suggested sounds bollocks.  But the female of the species does control the sex of her offspring to a certain degree.

Normally in humans there is close to a 50% chance of having a girl, with the number of boys born being slightly higher to compensate for the higher number of male baby deaths.  All foetuses are nominally girls so those with a male chromosome have to fight to exert this masculinity, resulting in more early deaths. 

The reason why the sexes are roughly equal under normal circumstances is because any significant inequality would favour the minority which became established.  Significantly fewer males for instance would mean  that some females might not get to mate and thus not be able to pass on their genes at the same time as all males would have increased chance of finding at least one partner.  Under such circumstances then it would be more advantageous to give birth to a boy.  If the population were skewed the other way many males would not get the opportunity to mate and it would become more advantageous to give birth to a girl.

There is some evidence also - about 70% of the children of US presidents have been boys - that high status mothers are better off having boys.  Such boys will have no difficulty in finding a mate and passing on their genes.  On the other hand, it is better for low status females to give birth to girls.  Girls cannot have quite so many children as is possible for a male, but they can usually find at least one partner. 

So if you want to increase your chances of having a girl go and live in the most execrable circumstances and with the lowest status male you can find.