Installing pipe. I’ve never heard it used as a euphemism for sex or pooping.
Kinky.
This.
Or this, actually. I actually worked as a pipe layer for a couple of years, so my first thought would have been “overtime.” The waggling eyebrows and the fact that he (presumably) wasn’t covered in dust and dirt would have tipped me off, though.
I’m pretty sure I’ve heard it used for both meanings.
Pipes are hollow.
When Jamie Spears (sister of Britney) announced her pregnancy, news reports reported the baby daddy was a 19 year old pipe layer. Obviously!
–Cliffy
An old joke.
Perhaps both!
I really don’t get the cable one. 
Poop is more flexible, and ropey, I guess? I’ve never heard it before.
I have heard of laying a some pipe, ya know, six inches at a time.
Me, too. I would take it literally and figure the person was doing DIY at their house or a friend’s house. I’ve never heard it any other way.
This.
As all of my esteemed fellow posters have already mentioned, I concur that ‘Laying some Pipe’ means one of any variety of pipe-laying…either electrical, hydro or otherwise drainage stuff.
Perhaps it’s because I’m from West Texas, but to me it means you’re an oil-field worker.
I even recall my high-school drama teacher telling us about how he laid some pipe in the Alaskan fields. My smart-ass buddy made an inappropriate joke about how that must have looked, which caught the teacher by surprise, because he’s not thought about any such connotation.
Poop can’t be pipe, because poop isn’t hollow. So really the only other “laying some ____” reference that could work for poop is cable.
Speak for yourself
I’m familiar with the expression “laying pipe” to mean “having sex”, but I’ve never heard “laying cable” before. The eyebrow waggle would leave me puzzled. I’d probably have thought it referred to masturbation as a less strenuous version of laying pipe, that could be managed solo 
I was similarly puzzled the first time I heard “pinch a loaf”. The image I had was of stealing bread, a la Les Miserables. And you know, I’d feel a lot more sympathetic for Inspector Javert if he was busy chasing down a known public defactor across the years.
I won’t bother to quote the previous posts, but I’ll pitch in which those whose minds jumped straight to the literal meaning of “laying some pipe”.
The height of the ‘laying pipe as slang for sex’ phenomena was the early '90s.
See New Jack City for examples.