If we try to follow the practices and we are careful with expressions..Can we say we are not just retaining the sexual energy?
Retention must be accompanied by radical psychological change, in which the transmuted energy nourishes the liberated consciousness and creates the higher vehicles. It is not easy to ensure that the energy only feeds the consciousness, since we have so many defects. We have to do our best, and get better at it every day.
Is the EGO called Karma?
Each ego (defect, desire, etc) is a material and energetic result of mistaken actions, which in turn attempts to perpetuate that action. So, each ego is a facilitator of karma, whether consciously or unconsciously. In other words, each ego is a mechanical device, a machine, through which flow the empowering forces that allow it to act. The machine facilitates an action, but it is not the action itself.
If the EGO ( desires, lust, pride ) is the responsible for every disease?
No. Read The Five Cause of Illness in:
http://gnosticteachings.org/books-by-samael-aun-weor/esoteric-medicine-and-practical-magic.html
Could the sexual energy retention assist for chastity ( if we practice, keep our centres on a balanced way ) ? Could the sexual energy feed lust ? (if we don't live the three important factors, do not practice, do not keep our centres on a balanced way)
Like any other energy, it depends upon how we use it. So yes, even if we are transmuting, it is very easy to then use that energy to feed lust. If we do that, it means the lust is being strengthened by a more potent energy, so it will have more force, and get even bigger than ever. Sadly, this is very common. Many people love to transmute, but very few seriously work on eliminating their defects.
From
http://gnosticteachings.org/books-by-samael-aun-weor/the-major-mysteries.html
This is how horns have grown on the foreheads of the Astral bodies of many bodhisattvas; this is how these fallen bodhisattvas have become demons. Therefore, from Gnosis one becomes either an Angel or a devil.
After a false step, many continue rolling downward into the abyss and finally the horns grow on them and they become demons. This is the remarkable reality in Gnosis.
For in much wisdom is much grief: and he that increases knowledge increases sorrow. - Ecclesiastes 1:18
Therefore, stay back, nosey people!
Therefore, stay back, profaners! Gnosis is a two-edged sword, it defends and gives life to the humble and virtuous; yet, it wounds and destroys the curious and impure.