Drinking, Kayla freely drinks with him and supplies him with booze.
Drugs, Nick is on drugs when he's out partying with Kayla.
Porn, Kayla is actively feeding Nick pictures of other women and participating in lewdposting.
Degeneracy, as they both don't recognize their non-monogamous proclivities as such, and are (at best) overly comfortable with including others in the sexual aspects of their marriage. This includes exhibitionist activities of a sexual nature, since the purpose is to include the public in what is private therefore making it naughty and fun (that's the whole appeal).
All of this is speculation as well, but it's speculation that's grounded in experience.
If I had to level a guess, my guess would be that alcohol is Nick's primary drug of choice. He'd be more than happy to just get blotto in front of his computer screen every night. I strongly suspect that the excursions into MDMA, shrooms and what have you are driven by Kayla, rather than Nick. It's my guess that she has experience of recreational drug use prior to her marriage -- but I don't believe this is true of Nick. A young man who has never had sex isn't going to be going out snorting coke, dropping molly and smoking weed. It just doesn't ring true.
But Kayla had been in a long term relationship. Of course her previous boyfriend was banging her, and it's highly likely that he was getting high as well. Now that her child bearing years are all done and she doesn't have to worry about giving birth to a brain damaged kid, why wouldn't she reminisce fondly about the days when her and Chad used to drop a tab and go out raving? I imagine they'd have been at college just at the time when the whole rave scene and associated drug use thing was just getting big in the USA?
I think Nick is definitely looking to Kayla for guidance and leadership in the sexual realm as well. Anyone who drinks as much as Nick does isn't likely to be that sexually adventurous. He's going to be lucky if he can get it up -- performance is likely to be limited to a perfunctory roll on/roll off mish posish. But Kayla will be at the peak of her sexual urges. The woman's got needs and they'll need fulfilling. Whether that's getting serviced by our bull Drex, some kind of sexual exhibitionism down at the local gay bar, swinging at Hedonism II or all of the above, we can only really speculate, but remember what Faran Balanced said: it's Kayla who wants to do the swinging. Not Nick.
I believed her then, I believe her now.
Kayla is ripe for a predatory man to come along and pluck her out of domestic tedium and into a passionate affair. I've seen no evidence that it's happened so far, but I think Nick is worried about it, and I think he's correct to be worried. Sadly, he lacks the moral and spiritual authority to correct her.