What can happen to me at work if I dated a co-worker?

First, if there is  a union agreement, you have to look at what that agreement says about sexual harassment, about intra-workplace dating, about discipline, etc. The contract or agreement will control any topic it has terms relating to.

Second, look to employer policies. Some employer's have no trouble with intracompany dating. Others don't allow it at all; if you your employer would bar the sort of relationship you had, you both could be fired (though again, subject to the union agreement(s)).

Third, sexual harassment in the workplace is banned. Depending on the facts (e.g. do either of you supervise the other; who said or did what?) you could potentially be seen as harassining him, just as he could be seen as harassing you. Whomever is considered to have harassed the other could face liability.

Fourth, to the degree your union membership does not protect you from termination in a case like this, you could be terminated by your employer simply because it feels that one or both of you are potentially disruptive  or liability risks.

In short, there is almost no way this can work out good for one or both of you. It's one thing if you truly were harassed in some way (e.g. pressured into a relationship by a supervisor)--then you should seek compensation and justice. But if this was simply a relationship gone bad, the workplace has nothing to do with it--why open that can of worms?

As for telling his wife--if you do this, there are situations under which she could potentially sue you (e.g. if you can't prove matters; for  defamation or some form of harassment or infliction of emotional distress); it's not likely, but it is possible. And when volitile emotions get mixed in, there's always the threat of retaliation in some form or another. Is it worth doing this--and also harming the wife, who may be an innocent person--in this case?