In Jain Dharma, not all Jains avoid sex.

• Sadhus and sadhvis follow brahmacharya completely, so they do not have sex.
• Shravaks and shravikas are householders. They live a family life, and marriage is allowed for them.

So Jainism grows mainly through householder families having children, while monks and nuns preserve the path of strict renunciation and spiritual discipline.

Also, in Jain thought, the goal is not to increase numbers, but to reduce karma and progress toward liberation. The growth of Dharma is measured by right faith, right conduct, and ahimsa, not only by population.