(also known as: appeal to egalitarianism, appeal to equity)
Description: An assertion is deemed true or false based on an assumed pretense of equality, where what exactly is "equal" is not made clear, and not supported by the argument.
If women get paid maternity leave, so should men.
Explanation: There are some good reasons that men should get some form of paid paternity leave, but they are not present in this argument. There is an unstated assumption that what benefits women get, for whatever reason, men should get the same. This begs the question.
Gay marriage should be the law of the land because gays should have the same rights as heterosexuals.
Explanation: What do the "same rights" mean? Before gay marriage, men had the right to marry women, and women had the right to marry men. A man marrying a man is a different form of the "right of marriage" and needs to be argued as such. Again, there are many excellent arguments for gay marriage, but this isn't one of them.
Why should unborn babies not have human rights, yet a day later when they exit the womb, they do have human rights? Clearly, unborn babies deserve to have the same human rights as the rest of us.
Explanation: This argument makes the assumption that unborn babies and babies outside the womb are equal, and ignores the fact that one is dependent on the resources of the mother's body to survive. Like with gay marriage, there are many excellent arguments for extending human rights to unborn babies, but due to the appeal to equality, this isn't one of them.
Tip: Don't get outraged when one questions what you believe should be "equal." Very often, either you really mean "similar" or you and your opponent have different concepts of what exactly should be equal.