The only definition that could be applied to the term "inhuman" is something "other than human", and if we need to use this term verbatim then we would be talking about sexuality among other species, say bugs, reptiles, etc.
If you mean "inhumane" then that would entail the abusive commission of a sexual act; one which is performed without consent (against the wishes of one of the parties involved) or without considering the sensibilities of one of the parties involved.
Human sexuality, itself, has been divided into seven categories
It is important to point out that, regardless on what judgement is placed upon one or another sexual practice, these particular ones ARE human.
Inhumane sexuality would then refer to practices perpetrated against others using the sexual activity as conduit or medium. Take for example, cases of rape, trafficking, acts committed against animals, acts against children, or the elderly. In most times, any illegal practice of the sexual act is considered inhumane because the law has taken it upon itself to protect others from the intentions of people whose purpose is to harm them.
The easiest way to discuss these kinds of realities is, first, to learn what they are, what causes them, and who is prone to commit them. Once you get to the root of what would be an inhumane sexual behavior, the shock and wonder that is associated with it will diminish, thus lowering the affective filter and allowing for new information to flow naturally.
When speaking of these topics, always have back-up information in the form of evidence such as titles of books and journal articles written on the subject. Also, keep the DSM-V, which is the newly-reprinted Diagnostic Statistical Manual of the American Psychological Association, which explains in detail the root of most (if not all) psychological disturbances that may lead to these acts.
To break the ice, speak about the effects of these kinds of behaviors in our current society: has anything happened in the last years related to this form of practice? How can learning about these behaviors, which DO exist in our world today, help us avoid a bigger issue?
Hence, information and transparency are the key elements to discuss controversial topics.