"I asked her why and she listed a lot of reasons, none of which I believe. She said it'll look weird in the photos, it'll be weird when Kate has to walk down the aisle with her maid of honour, that she doesn't want Kate to be distracting to guests, that Kate's too short to stand next to a bunch of 6ft+ guys," he wrote.
"The reasons were ridiculous, and I told her as much. She told me I should respect her wishes for the wedding and gave me suggestions for other Best Mans. I know the real reason might be jealousy, but I don't want to just drop it and go along with her insecurities."
He said they ended up having a massive argument before he left the house and slept at his mates' houses for a few days.
"Jas was upset that I wasn't budging on my stance and I was mad she was acting so irrational," he explained.
"She was being really accusatory about why I wanted Kate in the wedding so bad. I finally told her if she doesn't want to let me have this one thing maybe there shouldn't be a wedding at all.
"She got really angry and told me to just leave our apartment."
Since the argument, the groom has been desperate to talk to his fiancee, but she won't budge until he apologises.
However, he said he didn't want to as he still believed he had a right to choose his own "best man".
"And before anyone asks, no, Kate and I have never been sexual or romantic," he continued.
"She's been one of my closest friends and introduced me to my current social circle (I used to be an awkward nerdy guy who had trouble making friends, now I'm an awkward nerdy guy with a social life).
"We survived grad school together and she's been with me through high and low parts of life and it seems insane to not include her in my wedding. Jas and Kate know each other but aren't friends, and over time Jas got used to me having a close female friend without being jealous/mistrusting, or so I thought."
The man finished his post asking for others' opinions on who was in the wrong.
"This wedding is just as much your day as it is hers, and you absolutely deserve to have someone up there that you trust as your Best Wo/Man," one person wrote.
"Your fiancee needs to not only recognise that, but figure out whatever insecurity is causing her to lash out in this manner."
Another added: "My Best Man was a woman also. She was like a big sister to me, my best friend for many years. This is a hill to die on. This is really good time to have a major re-assessment why you want to marry this woman."
One user warned the man to consider his fiancee's behaviour as a serious "red flag", saying: "I'd put my foot down and not budge. The only compromise I can think of is that neither of you get a Best Wo/Man. Problem solved."
However, others sided with the bride-to-be.
"If it was my wedding my entire family is so extremely gossipy and judgy that they'd start whispering about it implying there's something going on," one person said.
Another agreed: "I really don't think it's insecurities as much as wanting a picture-perfect 'traditional wedding party'...bridezilla. But what will OTHER people think! And the pictures won't match man, woman...she said it herself...when she mentioned them walking down the aisle."
Another wrote: "By picking the lady friend, over the wedding at all, you've also sort of unintentionally validated any other concerns that she may have been withholding."
