Fire is only one aspect of Kundalini 
She manifests at various stages as Agni Kundalini (fiery), Surya Kundalini (solar), and finally as Soma Kundalini (lunar) …So, to call Kundalini as inner fire is not correct. Hatha (force) Yogis try to forcibly awaken Kundalini through Yogagni (the inner fire lit by yogic techniques) - so the fire is one potential precursor to awaken Kundalini, probably most known in the Western world. This path of the forceful awakening of Kundalini through yogic heat is inferior and causes problems.
While we speak of Kundalini mechanisms, as a universal rule, that is not true. One approach probably became more popular than the others, so it gets spoken of as the truth.
Don’t worry about your penis/testicles, you’ll be fine 
Try to do things you enjoy, distract yourself from thinking about this too much - or doing any kind of energetic manipulation for a while and enjoy the material bounties of life for a bit and this stage will pass