Unlike Abdication or Negligence, which are vices about disrespect of the resources of your partner, cowardly behavior is a character "vice of deficit" that sees one as depriving the partner of teamwork through disrespect of the self. A cowardly person certainly lets down their partner, but before that can occur, they make themselves look weak and ridiculous.
In being unjust as a partner at contributing to achieving a goal, the cowardly partner is letting their own fears produce the psychological impoverishment of the other partner. It is unfair and unjust, derelict enough to declare a direct contradiction to legitimately having joint goals.
People are often being cowardly, even while being encouraged, or being given confidence by their partner. It is as if all the confident effort by the heroic, courageous partner is going down the drain, and it is - through the holes in the boundary of the cowardly partner.
The cowardly use the most Immature Ego Defense, called, "Denial" - which is like "burying your head in the sand" and refusing responsibility or understanding of one's own actions, always blaming someone else or something else for our moral shortfalls. (The Ego Defenses y guide us in finding friends and mates who happen to have similar maturity levels.)
It is, perhaps, surprisingly pathologically narcissistic to be cowardly, because we usually us its opposite, given that it produces destructiveness, a "win/lose" behavior - causing "cheating," 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 Service as a virtue is called, "Martyrdom," where a person "plays the victim," 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 cowardly is a thief, directly countering the goals and dreams of a mate, by robbing them of resources in a deal which cannot possibly be fair.
As a vice which makes partnership within relationships fail to get to goals, and intellectually unattractive, consider cowardly behavior 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.