I think if you haven't unpacked and understood those unmet needs in therapy they can become a burden on a relationship (I still have no idea how H put up with my constant 'just leave me then, if I'm such a problem' tests when we first met). I don't think a partner can or should fill the role of parent because the relationship should be on an equal footing for it to work, however with aspects that therapy fills such as loneliness, need to feel valued and loved etc, I think therapy can begin to take a backseat in filling those kind of needs.
It certainly wouldn't be a case of 'I'm in a relationship now, I don't need therapy anymore' because that would put tremendous strain on a relationship.