TW: Where is the line drawn between "being drunk is not am excuse to cheat" and "drunk people cannot consent"

Asking again because my post got removed and I do not know why.

Scenario: A girl goes to a party without her SO. She gets drunk and meets up with another person. The other person is completely sober, and they have sex. Before, he asked if she was okay with this and was unaware of her SO. She wakes up the next morning next to the other person and realizes what happens. She tells her SO immediately where they say she should seek support for rape because drunk people cannot consent. Even though she said yes when asked.

Flip the roles

A guy goes to a party without his SO . He gets really drunk and meets up with another person who was sober and they have sex. He wakes up the next morning next to the other person and realizes what happened. He immediately tells his SO and they choose to break up because "being drunk is not an excuse to cheat."

Why is it often that when the girl in this situation, it's rape but when it's a guy, it's cheating? Where is the line drawn between not being able to consent and cheating?

Thoughts?

I've personally been affected by this situation. Am curious other people's thoughts