There was a time in my life where I would go at it at least once a day, sometimes more. And it would take very little mental stimulation to get me in the mood. A section of a book, a sex scene in a movie, some porn or erotica online, BOOM ready to go. For a while I was even participating in cyber sex, which was fantastic when you had the right person.
The frequency has gone down a lot over the years, I probably average about once a week at this point, with just my imagination, either remembering past events or just fantasizing about things. Partially because I have a wife now and can actually have sex with someone when aroused, but also because I have very young kids and am just constantly tired. I think Jeff Foxworthy put it best with his bit about the difference between single people and married people with kids. "When you're single and you are given the choice between sleep and sex, you'll take sex every time. When you're married and have kids, you take the sleep and just hope you have a dream about sex." It's so very true...
EDIT: Oh, and I have done my business in a few unorthodox places, including the car (both driving and parked), some rest rooms while out and about, and others that I'm not willing to disclose.