The 8 Limbs of Yoga - Brahmacarya

brahmacaryapratisthayam viryalabhah

At its best moderation produces the highest individual vitality

YS II,38 translation by TKV Desikachar

Brahmacarya essentially stands for the ideal of chastity.  There have been many yogis who practise chastity, but even more who have been householders, married with children (Mr Desikachar, Mr Iyengar, Pattabhi Jois, Krishnamacarya, Gandhi, among others)  This might be seen as having your cake and eating it, but for modern yogis chastity is a rare choice.  For most of us brahmacarya is about practising self-restraint, being faithful to our partners, avoiding using our sexuality in a way that causes harm to ourselves or to others (ahimsa).  It is using discrimination in our dealings with other human beings and seeking to live with wisdom and to practise moderation.