I thought I was an expert on this, have posted on many previous threads. But I started a thread last month about conception when having sex 7 days before ovulation, as it was I wasn't pregnant then, but lots of the answers I got were from mums on here who had had sex maybe 5 or 3 days before and still had boys. So blew my theory out of the water, so to speak!!
The other way of thinking, which has been used for a long time, is the acidity/alkalinity of the women when having sex, ie. if she has had an orgasm can apparently affect sperm much greater.
So for a boy you should orgasm before the man, and for a girl you must not orgasm at all.
I have been one stage further for dd1, and douched with a vinegar solution before hand.... you would be amazed that any sperm could live through that.
Last pregnacy was more tricky as we had boy/girl twins... can't explain except maybe girl was conceived on day 9 and boy on day 15.
Have never changed my diet for any of mine, think it is a complete waste of time, and wish I could help Victoria Beckham out, all that money and still she is not getting the right info!!!