when the sadhaka is firmly established in continence, knowledge, vigour, valour & energy flow to him
Yoga Sutra II, 38, translated by BKS Iyengar
Celibacy is not what is meant by this sutra, although that is what it is often taken to mean. Some of the greatest ancient yogis had families, as well as many from the modern era (Krishnamacarya, Iyengar & Desikachar for example); the great sage Vasista was himself said to have had 100 children.
Rather this sutra is an exhortation to treat our sexuality & sexual union with another with as much care, discretion & discernment as the rest of our yoga practice, and to a degree which matches our commitment to careful, considerate, gentle living.
Renunciation can of course be an act of devotion for a yogi, but is only an effective path when taken with a positive outlook; when performed with suppression & denial it can never bear the fruit of deep peace & joy that yoga practice brings.
It is a very simple sutra really: contain your sexual energy, express it wisely & with discretion; in this way you will gain vigour & dynamism; the rise of what is known as ojas: that vitality & radiant health (both physical & spiritual) that you see glowing in the faces of true yogis.