According to the Bible Encyclopeaedia and Scriptural Dictionary of 1902, in Biblical times,
adultery was the act whereby any married man was exposed to the risk of having a spurious offspring imposed upon him. An adulterer was, therefore, any man who had illicit intercourse with a married or betrothed woman; and and adulteress was a betrothed or married woman who had intercouse with any other man than her husband.
Personally, I would say no you have not committed a sin, but the fact that you feel that if the circumstances were real that you would follow though brings to your attention that you have a weakness you need to address. In your venacular, this is God showing you your weakness. That's how we refine ourselves. As our weaknesses are brought into the light, recognize them and adjust accordingly.
"Peshat is what I say and derash is what you say." --Nehama Leibowitz