Meditation and comprehension of your lust will aid you in annihilating desire and controlling the sexual act.
Also, be wary of developing too much fire that you eventually reach the spasm. Since as Hermes taught:
Separate that spirituous earth from the dense or crude by means of a gentle heat, with much attention
Gradually introduce yourself to the practice, gently arouse the flames of sex, and practice in shorter periods of time. Do not stimulate so much fire that you are already on the precipice of orgasming!
With time, you will train your body to retain and sublimate the energy for longer practices. I do not recommend building up so much passionate fire that you get burned. Gradually extend your sessions with time and patience, until you acquire stamina.