Button Fly jeans are easy, even when drunk. it requires the same skill that enables you to undo an attractive lady's bra with one hand, in the dark. (it's a jedi skill)
it's better than the chance of any zipper related problems.......nah mean?
Besides, you would only need to undo all the buttons if you intend to sit down to pee. - seriously,undo the top 2 and whip it out.