Why is it that when someone has one deviant interest, they tend to have more than one?
Because deviance is a choice they make when they cannot succeed directly playing by the hetrosexual game. If being gay is genetic then this whole thing seems very confusing, why do these deviancies cluster? However, paraphilia has been argued to be a learned behavior ranging from too many stronger boys growing up leading to mixed feelings of emasculation or inferiority which spiraled to childhood trauma which in humiliation they choose to cope and internalize. Its an aspect of their character to be the weaker effeminate, its part of their sexuality that they were molested as its easier to process if its not a bad thing but something they did to themselves by wanting it.
Is it a mental illness issue?
It is a learned behavior which alongside Dorian Gray syndrome, such as with Baked Alaska and Chaggot, presents as a form of cope for not being able to compete in the realms of bravery and strength so they compensate with dysmorphophobia or fixations on arrested development. Why do you think Baked is so confrontational one second yet flighty the next or Chaggot couldn't cope with thinking his 13 year old fuck-buddy was an evil thing?
A lot more of homosexuality is masculine sexuality unrestrained by femininity than hetrosexual men want to admit. However then there is the
Standard Fuck Party people.
"I'm not really worried about getting anything.. else.. right now cause I want to have fun right now and I don't want to have to worry about it" Which modern society can pretend are just like you and I but once you make a choice to be like that, its a matter of boredom fighting disgust reflex until you sexualize your disgust reflex and then - along with coping from your emasculation during your childhood - you just learn to chase degeneracy in order to "be carefree" and dependent-on-society. They often talk amongst themselves about disgusting society in order to further normalize their behavior, but only carefully and never explicitly.
You would be amazed by the percentage of their LGBTQQIP2SAA/LGBTIQCAPGNGFNBA culture which is just trying to disgust society while confusing sexuality with childishness. And its all learned behavior which we culturally indulge them in.
Is it a problem of being terminally online, as many of these types are?
The internet is a place where they can cluster more easily, but clustering together and feeling acceptance is what they are trying to do when they communicate on the internet. If elsewhere were better for doing that, they wouldn't be online much. However, it is the internet we're talking about.
Is it because having an interest like that leaves your mind open to more fucked up shit?
They hate normalcy because they can't have it, it mocks them. So they reject all forms of it in a childish immediacy which is a confusing mindset to relate to.
It has a lot more coding and careful wording in order to maintain the lies then you think. Which causes them to want to branch out and communicate with non-queer, but as their defense mechanisms immediately stand out socially, it instead causes them to have to form queer versions of everything. The queer versions of everything then in turn make queer become everything openmindedly.
Escapism/Disgust-chasing/Arrested Development all become very intermixed socially in gay circles.