The phenomenon Tracy is talking about takes the intrusion to a much more personal -- even abusive -- level, in which the victim feels sexually touched, caressed and even violated to the point of sexual climax. They feel that the spirit (a male spirit in the case of an incubus or a female spirit in the case of a succubus) activity is quite real since they have a physical reaction to it.
So what's going on here? As is the case for all such phenomena, no one knows for certain. All we can say is that the experience is either real (in the sense that the person is truly being attacked by some unseen force) or that it is imagined or psychological in nature.
Can it possibly be real? If we accept that spirits can interact with us, then we also have to accept that the incubus/succubus attacks could be real. If spirits of the dead can return to deliver messages and otherwise affect our physical world in ways that have been documented (we hear their footsteps, their voices, they move things, etc.), then unkind or disturbed spirits conceivably make such attacks. It is theorized by researchers that spirits reflect the personalities of the persons they were when they were alive. If they were good and kind people, they will be gentle spirits. If they were mean, violent people, their spirits might have the same attributes. So such a spirit might abuse a person sexually.
Religiously minded people might simply blame such attacks on demons.
We must consider, however, that such experiences could be entirely imaginary or psychological. The human subconscious is a deep and mysterious thing about which we know so very little. But we do know that it can be quite powerful. The subconscious can affect our health and therefore can result in physical changes or manifestations on our bodies. Parapsychological researchers suspect that the subconscious is responsible for much poltergeist activity. So it seems quite possible that a person's subconscious, stimulated by some deep-seated desire, fear or even past abuse, can generate the incubus/succubus experience and have it seem quite real -- even to the point of physical markings!
So to get back to the question: Are some people more susceptible to this than others? The answer, of course, would have to be yes since not everyone has these experiences. If it is caused by real spirits, the victims might be more sensitive to that world. If it is psychological, there could be many reasons why their subconscious would manifest the experience.