There is no known reason for humans to put their complicated bodies or their complicated sexual systems at risk in order to have an easy way out for resisting infidelity. Any medicine that reduces sexual desire is more than likely to disguise the real causes of the infidelity problems, and incur intolerable costs for either the committed mate or the individual who ingests the medicine.
The first issue about using drugs to resolve poorly understood or undiagnosed emotional problems concerns the side effects and possible catastrophic injury, illness, or death of the living entity. While new pharmaceutical products have been rigorously tested in limited and short termed protocols, the public is the real test subject of any true long term and broader testing of the product. This is when, after a medicine has been on the "market" for a time, the serious side effects and long term physical consequences are discovered. And the pharmaceutical firms include a certain amount of "acceptable risk" in the form of serious medical consequences or death when they push a product through the FDA approval and marketing process.
"Acceptable risk" is not so acceptable to the family and loved ones of a person who is permanently damaged or who dies from the misapplication of a drug. And a drug that suppresses sexual desire is just that: a drug. The drug does not deal with the true origins of the problems, such as sex addiction, dysfunctional emotional processes that lead to promiscuity and infidelity, and other issues that are the root cause of the desire to stray from the committed relationship.
Perhaps the relationship itself is sick, and therapeutic psychological couples counseling would help to correct the problem. Suppressing the sexual desire of one could be the result of a controlling and abusive partner having their way. What if the partner is the problem, through paranoid or other maladaptive behaviors, and there is no infidelity in the first place? If the other partner is weak and compliant, they will submit to just about any form of controlling and abusive demands no matter how much of a risk they are taking that their own health or well being is at risk.
It should be a legal, medical and ethical requirement that before anyone could even get a prescription for such a drug, they must undergo complete physical and psychological testing to determine the root cause of any out of control sexual desires. If the issue can be treated or resolved through therapy or counseling, or even putting an end to a problematic and dysfunctional relationship, then the individual should be denied any medication as a treatment of something which has not been properly diagnosed, and where attempts have not been made to resolve the problem through other means.
Such medications, however, should be an option, where not life threatening, for repeat convicted sexual predators and offenders who are released back into society, and hopefully the laws will be improved to prevent the repeat offender from being released back into society, negating the need for any drugs.