From what you posted I think you will be hard-pressed to prove that you were discriminated against based upon being a male. You were likely an "at will" employee. This means you could be terminated for any reason, or no reason at all, at any time so long as it isn't based on discriminatory reasons (i.e. because of race, sex, national origin or another protected classification), or your having engaged in a protected activity, or in violation of the terms of a contract or collective bargaining agreement. Being “at will” also means that the employer is not required to provide advance notice of a termination decision and is not required to justify the decision with “good cause.” If you think you can prove there was discrimination based upon your being a male, consult with an employment law attorney, but otherwise, while your termination may not have been fair, it doesn't sound like it was unlawful.
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Reading between your lines, it strikes me that you do not believe the reason they gave you was the real reason you were fired. Your statement, "Just any reason to fire me", is very telling. Clearly, you know there is something else going on here. Was it because of the bullying investigation, a personality conflict or something else? The answer to the question, why are they really firing me, will go a long ways toward figuring out if any rights have been violated.
If you take anything away from this, know that you cannot sue an employer for lying to you about why they fired you. In fact, true or not, most employers have the right to fire at their will, which includes an unfair reason. Getting fired for drinking with your buddies may be hypocritical but it is not illegal.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
Double standards in the workplace are common. In fact, discrimination in many forms happens all the time, such as favoritism, nepotism and the like. The difference in treatment is only illegal if the reason you are being treated differently is because you are a member of a protected class of people, or because you have engaged in some form of protected conduct. Your post does not suggest such a motivation.
Life is sometimes not fair. It is unfair you were treated differently in this matter. However, it is not unlawful.
Good luck to you.
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As you state, your employer was looking for "any reason" to fire you. Fraternization is simply the reason they happened to choose. Doesn't sound like gender discrimination to me unless you are omitting relevant facts.
This answer is a general interpretation of the law and is not fact specific to your case. Likewise it does not create an attorney-client relationship. You should seek an attorney for a review of your specific facts and documents. http://www.johnphillipslaw.com