男生有大姨夫才公平

In Chinese culture, this is a humorous way to refer to PMS, indirectly suggesting 'men should have periods to experience women’s menstrual discomfort', implying a form of 'fairness'. But note that some people may use it with a different context, and the expression itself can be quite specific to internet slang and cultural understanding. For many non-native Chinese speakers, this concept could need extra explanation beyond a literal translation.