Moral trump card

Regardless of whether they admit it or not, any system of ethics has an implied ethical calculus.

Anything considered an absolute moral good or evil within an ethical system, where it is unacceptable under any circumstances, can be considered to have infinite moral value in the utilitarian ethical calculus.

The presence of infinity really messes with your equations, as anyone familiar with math knows. In reality there are no infinities in utilitarianism because any action can only have a finite impact on human happiness. Except maybe blowing up the universe. Destroying the entire universe would be infinitely bad. Everything else is finite.

In the quality of life versus quantity of life moral quandary, the notion of "thou shalt not kill" is the quintessential moral trump card. If the fetus counts as a human life, then abortion is murder, and you can't beat murder so we take the trick.

These moral absolutes often lead to morally outrageous outcomes while bolstering the believer with a false sense of righteousness. By playing the trump card, they appear to be taking a principled moral stance, when they actually just lack nuance.

For more card-based metaphors see The Gambler.