One of my pc's, a male follower of Raven, managed to jump on the back of a unsuspecting unicorn. He's presently getting scolded by his companions and they all seem to think he will lose his fertility for the act. His girl back home is already pregnant so he's lucky on that side.
A classic spur of the moment craziness on his part.
As for 90% of Yelornans having unicorns? A high number but I reckon the heroquest is important and heavily supported by the Tribe. The more Unicorns the better and each one contributes to the Wastes returning to what it once was.
I imagine that when powerful unicorns walk, the ground under their hooves transforms momentarily to being fertile. Get a large enough herd and with some powerful quested magic I think a hidden Green Age field could be made or an Oasis made more fertile etc.