Methane and sulfur compounds are stuff that comes out of your butt. It's basically a fart. The methane is the loud part of the fart, while the sulfur compounds are the stinky part. The more sulfur compounds it has, the stinkier the fart.
When your dick is so dank it smells like sulfur and/or tastes like sulfur. Often developed through poor hygiene or sweat from busting your ass after a long hard day of work.
Boyfriend: Baby I know I worked all day but I need a blow job.
More scientifically accurate, Potassium Yttrium Sulfide, this spells KYS. If somebody tells you this, they either respect you enough to shit on you or really fucking hate you.
Doctor: I think you're retarded.
Retard: What can I do doctor?
Doctor: One 10mg pill of potassium yttrium sulfur should besufficient.
Retard: thx doc!