Excellent! I was just curious because of your terminology.
I would explain it like this:
Demons seek to inhabit flesh and blood bodies for many different reasons. One is to fulfill their hateful desires, but another is to temporarily and in a diminished sort of fashion replace the lost presence of God, which they can't feel anymore since they have been banished from His very presence.
When a person sins, it opens a type of doorway inside of a person. As one sins more and more, that doorway opens wider and wider. Demons can recognize it, as well as some who have the gift. But even when a demon enters this doorway and resides within a person, it can't just do what it wants. It or they still needs help. The stronger or more numerous a demon they are, the less help they need.
For instance, they'll create or take advantage of situations which result in great emotional distress or outbursts. Or when a person is highly medicated or on illicit drugs is another way. It's during these times that a person loses control of themselves and the demon can take over and function through the individual. Eventually the person regains control, but each time it happens it gets worse and worse.
When it comes to sex, you're right on there. The only thing I'd add is that the scenario your describing is what can happen between unbelievers, or more accurately those who aren't born again.
As for those who are genuinely born again and Spirit filled, they can't be demonized internally unless the Spirit has departed from them. This is the added step in a believer being demonized, which does happen.
When using the terminology 'the flesh', which is typically a religious term, that could mean many things according to one's religious upbringing. Whenever people use that type of speech, it sounds like it's simply theory coming from some book or teacher who may not have any actual experience themselves. You'll be able to reach a wider audience and be more precise of you let go of religious terminology ya know?