First off, it's not strictly defined what "co-dependence" even is, which is probably part of why you're having trouble getting a clear handle on it.

>Codependency has no established definition or diagnostic criteria within the mental health community.[14][15] It has not been included as a condition in any edition of the DSM or ICD.
>
>-https://en.wikipedia.org/wiki/Codependency#Definition

The reason it's considered "unhealthy" is (in it's most-commonly-understood meaning) because it tends to mean that one partner (romantic or not) is heavily dependent on the other for their emotional or practical needs, while the supporting partner exhibits excessive suppression of their own needs and emotions, and excessive self-sacrifice in service of the dependent partner's needs.