Unlike Resignation or Abdication, which are vices about passiveness that avoid responsibility by your partner in doing the real work of being a partner, Negligence is a character "vice of deficit" that sees them actually pretending and claiming to do the tasks of being a partner, when in fact, their activity remains self-centered, and to the ignorance of the other partner's rights and boundaries.
In being unjust as a partner in contributing to achieving a goal, the negligent partner is pursuing their own life's goals - their "mission in life" or "purpose in life" to the neglect of that of the partner. It is unfair and unjust, causing the other partner to start to perceive of "being used" while "being lied to," underneath the false pretense of being aligned toward joint goals.
People are usually being negligent unconsciously, in the codependent personal boundary state, possibly using the most primitive and childish Ego Defense of all, called, "Denial" - which is like "being caught red-handed" and refusing responsibility for one's own actions, always blaming someone else or something else for our moral shortfalls. (Learning about the Ego Defenses, you will find them to be a handy, objective measure of a personal character maturity level.)
It is pathologically narcissistic to be negligent, of course, given that it produces destructiveness, a "win/lose" behavior - causing what you might call, "bailing," or "checking out of the relationship," all the while pretending (even to one's self) as if they are actually participating in the relationship as a real partner.
The other vice of Justice as a virtue is called, "Meddling," where a person "takes charge," boldly and ignorantly attempting to define and constantly redefine what is just (and employing the Defense Mechanism of Denial as their strategy for constantly revising what is "fair" between them.
Someone who is romantically negligent, is directly countering the goals and dreams of a mate, and shows poor boundaries with a lack of respect for the good faith efforts of the partner to be equal and fair.
As a vice which makes partnership within relationships fail to get to goals, and intellectually unattractive, consider Negligence to be negative in many ways, including in sexual attraction, since it actually depletes the reserves of one's own, or the other partner's masculinity or femininity by robbing them not only of teamwork, but countering the personal dreams that empower the passions, making both partners less attractive, and feeling less attracted back.