I have been married in a civil union for 15 years. My wife is divorced, hence a civil union. This marriage has been misery. I have struggled with porn and lust. I met a ex-girlfriend for dinner once, twelve years ago. My wife claims that is cheating. Is this true?
Deliberately choosing to use porn and to fantasize about other women certainly counts as cheating. Whether or not meeting your ex-girlfriend for dinner counts as cheating depends on the nature of the meeting. Did you do so because you were open in some way to resuming your relationship with her? Did you do so because you were miserable at home and wanted to be with a woman who was not your wife, even in a non-sexual way? That might well count as cheating.
Whether or not your marriage is valid in the eyes of the Church is not the issue here. The issue is with whether you are engaging other women for companionship, either sexual (in the case of porn) or social (in the case of the ex-girlfriend), at the expense of your wife. Your duty to her is to do all that you can to regularize your marriage and to heal your relationship with her, if at all possible. But, at the very least, you owe her the respect and dignity of avoiding getting involved with other women so long as your marriage to your wife continues.