Although the sex and any singular sinful activity can not exactly be pin pointed as the primary source for your demonic possession, those activities can also act as hubs or windows of opportunities for demonic attacks.
Thus being said, it is important for you to stop your carnal desires to rule your body.
Like the post before me said that when God comes over and clean up your spiritual house the demon has fled.
However, if the demon come back and found that house is empty, he will get in to the house again; but, this time, he'll bring reinforcement.
You said you have accepted Christ completely?
But, how can this be?
From your thread, it seemed to me, you still live the way worldly people do...
In Matthew 3.
The greek version used to describe repentance is Metanoiete
meaning that "a 180 degree turn from our sinful life and abhorring our past sins and turning away from them."
Also within the verses, there is a singular word called 'poiete'
meaning to straighten the path and to commit.
You cannot repent one day and be sinful the next month and repent again. You have to stay committed to your new life in Christ. You might fail once every so often, but that should not be an excuse of wallowing in that life style.
Another word to describe repentance in the chapter is 'karpon kalon'
meaning that one should bear the fruit of repentance.
or in simple word, there should be proof of changing in one's life.
no more living in cohabitation, start in ministry, change in behaviors and so on..
Repentance that does not bring the fruit is a lie.
So, hopefully, this should be a reminder for ALL of us (including ME) that we cannot play around with God.
How can we say that we have Christ in our life, if we refuse letting go our old ways?