That can shatter the pillar of trust that a relationship is based on. I've experienced it before: one meaningful lie and no matter how many years of truth follow, the relationship dynamic is altered forever. There's always a cloud of doubt.
I think context is probably very important, but I don't think context alone can diminish the effect this kind of lie can have. You just kind of leave someone in ontological shock about the foundation of one of their most important relationships.