http://www.slang-dictionary.org/English-Slang-Dictionary/lay_pipe

You are getting a tad risque with your questions today, aren't you Nozturk?

"To lay the pipe" is to put pipes (water pipes, sewer pipes, electrical conduit) somewhere, whether it be underground, on the ceiling, or on a wall. "It's too wet to lay the pipe today." = "The ground is really wet, so we can't dig up the ground and place the pipes." "We need to lay 20 feet of pipe on the wall." That's the general meaning. If you're asking about something different, can you provide a context for the phrase?

it means to have sex

If your not at a construction site... it means to have sex.