Over the 200k years man has been around, the last 100 years has shown the Y chromosome shrinking. It is incredibly smaller than the X chromosome. Part of this is due to biology. During conception, the egg and the sperm begin comparing DNA and each taking out what the other has better. But there are situations where the X chromosome does not have a suitable replacement for something in the Y chromosome, thereby taking out that DNA segment of the Y chromosome and not replacing it thereby shrinking it. This has gone on for countless centuries and over time, has effectively dwindled the Y chromosomes power (there are many biological factors resulting from this but that is another discussion). For females, this is a non-issue, since they are XX so whichever DNA segment is bad they can just replace with the other's.

This is important because it tells you the size of the sperm. X chromosomes have not suffered from being ripped apart and left to pieces, since it's constantly adapting. Thus, the X chromosome carrying sperm is much larger than its Y counterparts. Size is critical for survival within the acidic tunnel of the vagina. The Y sperm does not last as long inside of the vagina than the X sperm. But nature has a funny way of balancing things out. Due to the smaller size, Y sperm is also considerably faster than the X sperm. Thus, like any RPG game, you have offense VS defense. The key part in all of this is timing. We then get into ovulation, and the release of the egg timing with the arrival of the sperm. Sperm is very small. To it, the vagina is miles long and it isn't easy to move about inside of one. The techniques I refer to are essentially favoring the bulkier X sperm through the passage. Namely, having sex before ovulation begins so that the Y sperm dies off, climaxing at the base of the vagina so the Y sperm has a long journey to reach the egg thus increasing odds of dying, and eating a fruit diet a month in advance to increase acidity