Is it legal to fire someone for dating an associate who makes a complaint?

I made a complaint to management. My supervisor wrote me up for "having a bad attitude". He didn't beat around the bush and told me directly that the reason I was written up was for using the open door policy. So, naturally, I reported retaliation to the store manager. The very next week, I had two write ups (I had worked there for over a year without any disciplinary action until I made the complaint). One was for one aisle that was supposedly unstraightened. Two of us were asked to straighten it, the other guy relaxed at the paint desk while I fixed it. Supposedly, it looked bad the next morning. The other guy closed, I did not. I got written up, noone said a word to the other guy. The other was for being 6 minutes late. I have never received a warning for tardiness or for keeping aisles disorganized. Yet, they fired me and my girlfriend for this (and made an accusation to HR that I started a petition, which came out of nowhere as I have done nothing that could even be confused as starting a petition).

My girlfriend not only had no part in these complaints, but didn't even know I made them. She had worked her way up to DH before stepping down voluntarily, never had a write up, and was in good standing with everyone in the store (not exaggerating).

I already reported this to the labor commission and they are investigating, but I'd like to hear your opinion.