Yep, we believe it. Knitting your boyfriend (or girlfriend) a sweater can doom your relationship. It's happened to us - twice. We don't know how the curse became a thing, but we're pretty sure the knitting gods created it.
Maybe the curse is to warn knitters not to invest too much in a relationship until the commitment is there on both sides. Maybe it's that we knitters are too darned nice.
Whatever the reason, we take it seriously. For us, we don't go further than vests without a lifetime commitment :)