It is not unusual for sexual energy to come up in breathwork. It does not happen all the time but, if you just think about it, your sexual energy is your life force energy. It is the strongest energy in your body. It is the energy that drives you to procreate. So, from an evolutionary basis clearly it has to be your most powerful energy. So, when you are opening yourself up, sometimes that energy does present itself. If it does, just allow it to process through in the same way you would any other breathwork experience.