Unlike Tyrrany or Usurpation, which are vices about a takeover of the resources of your partner, Mercenary behavior is a character "vice of deficit" that sees the partner actually taking from each other in plain sight.
In being unjust as a partner in contributing to achieving a goal, the mercenary partner is pursuing their own life's goals - their "mission in life" or "purpose in life" to the direct psychological impoverishment of the other partner. It is unfair and unjust, bold enough to declare a direct contradiction of legitimately having joint goals.
People are often being mercenary unconsciously, and so when their behavior is pointed out to them, they are offended that what seems a fair deal to them is unappreciated. They use an Immature Ego Defense, called, "Rationalization" - which is like "always having an excuse" and refusing responsibility or understanding of 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 mercenary, of course, 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 mercenary is a thief, directly countering the goals and dreams of a mate, by robbing them of resources in a deal which only looks on the surface to be fair.
As a vice which makes partnership within relationships fail to get to goals, and intellectually unattractive, consider Mercenary 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.