What is the purpose of the monastic celibacy?

The sangha was established by the Buddha for those wishing to devote themselves single-mindedly to his path of awakening. The Buddha designed monastic life to be one of radical simplicity, with the minimum amount of unnecessary distraction. Romantic attachments, sexual relationships, and their usual outcome –parenting- are all incompatible with the training he devised. They would also compromise the symbiotic relationship between the mendicant order and society at large which the Buddha envisaged.

The Buddha discovered that the more subtle forms of happiness and the experience of true well-being are rarely accessible, and always unsustainable, while indulging in sense pleasures. He insisted that the Sangha be celibate in order to allow monastics the opportunity to investigate the sexual impulse as a conditioned phenomenon and learn how to relinquish identification with it.

The Buddha revealed that with spiritual maturity the sexual instinct finds no footing and fades away. As sexual desires, perceptions and thoughts are not only a hindrance to liberation, but owe their existence to a deep-rooted ignorance of the way things are which monastics are dedicated to eliminating, they choose to lead a celibate life.

Is it not unnatural for monastic to be celibate?

Yes, if “unnatural” means acting in ways that override the most basic human instincts. But it is generally accepted that human civilization has evolved to its present level precisely because human ability to go beyond the given, to be intelligently “unnatural”. It might also be argued that the human aspiration to govern basic instincts is experienced in the mind as naturally as the instincts themselves are experienced in the body. The sexual drive is perhaps the strongest human instinct and learning how to relate to it in a skillful way is a great challenge. In the Thai Sangha, monastics who find themselves unable or unwilling to sustain a celibate life return to the lay life without shame or criticism.