EDIT: Now that the jig is up, this is totally not real. Just FYI

It's interesting that you touch on this! Tea became such a largely traded commodity in part due to domestic and folk customs that arose during the Tang Dynasty, word of which proliferated along the western trade routes.

In pre-modern times, lubrication in intercourse was often sought after as a means to increase fertility, and we see many civilizations coming up with their own solutions and remedies (for example, some of the Mediterranean civilizations would use olive oil or other oils). During the Tang, as agricultural cultivation grew to unprecedented levels and the region enjoyed all time high levels of prosperity, tea spread around the empire to even the lowest levels of society. Tea and tea drinking often took on mystical associations with scholars and peasants alike at the time, and so it was reasoned that it may aid in fertility and strengthen any progeny.

At first, there were several tea ceremonies that were customarily performed for and by those wishing to guarantee strong offspring (in fact, you can still run across it in certain place in modern Beijing, performed more or less the same) and was simply limited to consumption of the tea by drinking. As the practice became more established however, more intricate methods and ceremonies were devised. A quote from the Yiwen Leiju compiled around this time accounts one of these practices:

So, [the] husband steeped his extremity in the tea for a period of between one and three days, allowing the Qi of the substance to enhance his own, and aid in providing thousands of offspring, each with the strength of tigers. [The] wife, similarly, draws a bath of the tea, and steeps herself for a period of one day, so that [the] child may develop and flourish