An event that is single sex is single sex for a reason. Upthread pp gave an example of a run to raise awareness of testicular cancer. Another good example is women’s breast cancer awareness.
As you may know, testicular cancer only affects men, therefore it would be perfectly reasonable to create a men only event.
This event was a family fun run.
Women are generally an important family member.
Reportedly some or the more religious men in the mosque said they didn’t want women there. So the organisers banned the group of females they deem to be ‘women’ ie all females age 12 and above. This says a lot about their view of women and their right to participate in ‘family’ events.
It is neither lawful or moral to exclude women from this event in the say so of some of the very religious men who believe that women shouldn’t participate in society on a level with men.
People, especially women , who think it is ok to treat women badly because of religion need to give their heads a big wobble.