I feel very different about this.
I recall having a pulled muscle. I knew it was a pulled muscle as I had a tightness in my chest and tingling. This happened when I had a lot on and it then occurred it might not be a muscle pull but something more serious.
What I did next was nothing. Had I been single, I would have gone to the doctor, but because I was in a relationship it would either be something serious, in which case the "manflu" thing would have added to my stress and likely made it worse. Or it was nothing in which case I would still get all the "manflu" stuff for nothing.
It actually was a muscle pull. I saved the doctor's time, but I am sure other men have not been so lucky.
On a more trivial basis, it is poison to a relationship. I recall when I cared for my SO other when we both had flu, going out and getting food and groceries when I would rather have laid in bed and done without, but she needed stuff. It was decades ago, but I remember her even now her complaining to her friends about how she had to care for me while I whined.
In contrast, when I moved abroad I had a SO wonder why on Earth I was covering up feeling unwell and my current wife accepted I was unwell and even brought me porridge for breakfast rather than seeing I was off and suggesting chores. That is a golden memory.
Also, there will be some men who really are whiners and abusers and sometimes their SO will not know it is not normal.
I expect to be told my perception is off, but I recall a Danish SO telling the British women who were having a conversation like this one(we were visiting) that I was very stoic when ill and did not make a fuss. She came to me wondering why she got poisonous looks.
I can see it is easy to feel let down or even betrayed when your man is unwell, but this is part of British culture that I am glad to have left behind.