what does ‘“Think of water under pressure in a pipe. I pierce it once, what a jet!” he pierced it twenty. There were twenty piddling little fountains.’ mean?

In this context, the fountains represent numerous sexual partners, as opposed to the pressures in having only one. Multiple sexual partners relieve stresss and allow for detachment. Attachment to one partner is considered unhealthy, as it promotes temptation, loneliness, and isolation.