Titles usually set our expectations for how to read the poem. But sometimes they can trick us, and when they do, that's usually the point. This poem has one of those tricky titles. "Living in sin" is a popular phrase that refers to living with your partner outside of marriage—like "shacking up." Usually it's also referring to a pretty exciting and steamy love affair, so that's what we expect the poem will be about. But we readers, like the woman in the poem, soon find out that shacking up with her partner wasn't all she expected it to be—it was definitely not dreamy or steamy.
So why is the poem called "Living in Sin" if it's not going to be about an idyllic relationship? Well, on the most basic level it describes the relationship accurately, in that we are to presume that the couple lives together outside of marriage. But the idea of a blissfully perfect romance doesn't work in this poem. Maybe there is another meaning we could consider? If we had to identify a "sin" in this poem, what would it be? We think one of the major possibilities could include the fact that the woman remains in the relationship at all, since it doesn't seem to be a happy one. Or maybe it's that neither one of them puts a great effort into the relationship. Either way, it's clear that—for the woman at least—living this way is kind of a depraved existence, which is what "sin" describes at the most basic level. Bad times.