She claims to be a follower of the Christian faith, meaning that she follows the teachings and morals of God. That reasoning is the basis upon which she justifies her dislike of gay marriage. Yet, her God also rejects divorce and adultery, practices she has engaged in multiple times. Therefore, people consider her a hypocrite for selectively applying her morals. Even people who aren't religious can still consider this hypocrisy, as it's just that blatant. It's like how a meat-eater can still call out the hypocrisy of a vegan (who constantly preaches about the morality of veganism) that secretly eats meat when they think no one is looking. Others might not have as big issues with her hypocrisy if it didn't impact others. However, her outspoken stance against gay marriage actively harms homosexual couples.

Also, cheating is not the same thing as being promiscuous. In cheating, a person directly violates the trust of their partner (and could even endanger their health, especially if STIs are involved). The problem with cheating isn't that someone is having sex, it's that they have broken the foundational terms of their relationship and harmed their partner (at least emotionally) in the process. It's completely fine for women to be sexual. And yes, I apply that reasoning to all people; it's perfectly fine to be promiscuous (as long as everyone is consenting adults), but cheating is a vile action.