From my experience with married "straight" men, sexless marriage is often the key driver for them to explore the ignored/suppressed/forgotten/whatever curiosity of male-to-male actions. (Yes, I love making straight guys stray...)
Someone I met through craigslist put it: "I used to pay a whore from time to time, but I don't have much money to spend. It's way better to have another guy who knows how to make a cock feels great to work my junk than to jerk off to porns."
I mean, who wants to eat McDonald's when you have a great cook at home? But then when there's no food at home and you go really hungry, the quick food at McDonald's starts getting enticing...