"Demisexual" refers to an individual whose sex drive is entirely dependent on an attraction or lack thereof to a specific person. In the absence of an attraction to a specific person demisexual is for all intents and purposes asexual.
A single demisexual may or may not masturbate and/or enjoy stimulation and or orgasm but forgo the pursuit of sex. Some may only have sex simply to please a partner. Some might enthusiastically be involved in a sexual relationship with that person, but in the absence of relationship does not pursue sex at all.
In a binary world where we understand a lot of things as all or nothing the notion of a switch that turns on and off for reason others can't fathom may be confusing.
It may be distressing or hurtful to some to realize that their partner has a sexuality that is very passive and reactive. Culturally women especially are taught they are inherently attractive and that not being desired by a partner means they're ugly and worthless. Culturally men are taught being able to score with a woman is a function of their identity. Culturally men are also taught to aggressively pursue women which may cause some conflict. Likewise people are likely to interpret a woman who has no overt sexual interest to be prudish, frigid or broken.
Your mileage may vary. Some might appreciate a partner with literally no desire to stray. Others with a more dominant sexuality might appreciate a sexuality that is responsive not aggressive. This is why communication is essential. But that is true of every couple.