What can my boss actually say to me? Written up for fraternization with fellow employee. Policies are vague at best.

I was written up for fraternization after dating someone in a different department. Neither of us is a supervisor and our jobs are unrelated. The policy is vague, I have now seen two different versions. One states that if you are in different departments it is okay. The one I was presented with yesterday stated that if you are in the same building it is not tolerated. The relationship is over, and we supposedly both had to sign a form that says we will not have any further "personal interaction." Vague much?

When my boss found out, he stated that he would turn a blind eye and that he could not control what we did outside of work. He then occasionally harassed me about the situation, in a "friendly" "teasing" way. As soon as it was over, he decided it was affecting work performance and wrote me up for it. In reality, him crossing a line and telling me that the other party involved never had feelings for me, used me as a rebound, and wanted nothing to do with me was what frustrated me enough to be visibly upset. The end of the relationship was amicable, and we do not work together as it is so there would have been no problem had my boss not gotten personally involved.

Furthermore, the fellow employee that I was romantically involved with currently resides with our boss. When I asked for proof that he was reprimanded equally the manager refused for privacy reasons. We were not counselled together and I have no way of knowing whether favoritism is being shown. The fact that the general manager lives with a subordinate seems more problematic than me dating an equal who works in a different department.

The discrepancy in policies/handbooks available also seems fishy. I don't know what my best angle is here, but I know something has to help protect my job if they try to use this against me.

Advice? Thank you!