I've been reading a very interesting book, Heroes, Rogues and Lovers by James McBride Dabbs with Mary Godwin Dabbs, on testosterone. One interesting finding in the book is that people with high testosterone have more sex overall and have more sex with different people. However, while high testosterone is related to greater amounts of sexual activity, it also works the other way around: Having sex also increases testosterone.
One of the most popular timing methods for girls is frequent BD, based on the assumption that more BD lowers sperm count. It may very well be true that lots of BD lowers sperm count. However, as I've studied charts, I have NOT found that frequent BD favors girls. In fact, frequent BD seems to be close to 50/50 or to slightly favor boys. Even extended frequent BD (at least 7 BD times in the 10 days prior to through the day after ovulation) was also pretty close to 50/50, just slightly favoring girls. And the charts with the most BD (12-13 BD times) significantly favored boys (4 boy charts to 1 girl chart).
Before I started looking at charts, I assumed frequent BD would favor girls. It was surprising to find that it didn't. After reading what I have about testosterone, though, I have found a possible explanation. My hypothesis is that frequent BD raises testosterone levels in the dh and dw, and that increase in testosterone (which sways boy) offsets the benefit of lowered sperm count (which sways girl).