The general belief is kundalini activates when sexual energy is transmuted towards the only goal of life. This is partially true as physical aspect of celibacy (abstention from sexual indulgence) constituted hardly 10% of total job. 90% celibacy is always practiced mentally.
For a soul atman to reach stage of absolute purity flow of thoughts to brain has to be stopped in totality. Practicing absolute celibacy human beings accomplished this. Eradicating flow of negative thoughts to our brain in totality is what absolute celibacy all about.
A combination of both celibacy (practice of Brahmacharya Vrata) and absolute celibacy which is exercised mentally is needed for Kundalini to reach its highest potential.
Unless human beings, seekers indulged in Celibacy mantra, nothing worthwhile could be achieved on path of kundalini awakening. Celibacy mantra demands that seekers maintain only three relationships with womenfolk on mother earth, that of mother, sister or daughter. There never was a fourth relationship. In the circumstances how could one indulge in sex?