There are many types of sex crimes. Three of them are:
Sadism is where one’s exercise of power over the other is mostly manifested through sex. Possible traits of a sadist include alcoholism, total control over or domination of a victim to an extent of humiliation, complete lack of shame or guilt, performing violent sex against the victims (Holmes & Holmes, 2008). The victim may die in the hands of a sadist. Because the sadist feels nothing at all, he/she may not know when to stop; it is at the subjection of others that he/she finds pleasure. But the practitioner is also dangerous to him/herself. Sadist has no emotions, no guilt of remorse, does not care much about him/herself either.
Infibulation involves the attainment of sexual pleasure through pain such as torture or other masochistic sex behaviors e.g. such as when one pierces the flesh of his/her own genitals (Holmes & Holmes, 2008). Here, there may be accidents with piercings during the sexual act with a partner.
Pyromania is an eroticized or rather sexually-motivated form of arson. That is, the practitioners attain sexual pleasure at the sight of smoke or fire (Homes & Holmes, 2008). A pyromaniac cannot, at times, stop himself from the act of arson. The urge becomes too strong. At such times, it is likely that their primary sexual hunger comes first. They may even get burned in their own fire(s).
I think the escalation of these violent sex crimes depends on the ability of offenders to get away with them. For instance, victims of sadism are not likely to speak up perhaps because of the fear for stigma or the fear of getting hurt more by their tormentors.
Generally, these crimes are helped by the fact that sex is quite private. It is not very much a public act that can have many witnesses confirming it.