Is fantasizing about cheating the same as cheating?

My childhood friends and I recently caught up over a few drinks. We ended up playing truth and dare after getting tipsy...and one female friend ended up confessing that she keeps an erotic diary that's filled with dirty desires she has for attractive men she comes across. She reasoned that writing about it gives her an outlet without acting on it. While I didn't want to cause any trouble that night, her explanation didn't sit right with me. If she's having recurring thoughts about straying out of the marriage...can it be considered just an outlet? While I'm not here to judge... I'd really like to understand if there are other perspectives to this.