How common is it for geldings to have stallion like behaviors with mares?

My gelding is the sweetest guy to humans and other horses, never any stallion like behavior. My trainer thought it would be a good idea to find him a turnout buddy, she picked a mare who he’s interacted with through a fence with no issues whatsoever.

However, when they were put together, he acted like a stallion. This was the first time he’s ever displayed any type of stallion like behavior.

Many of the mares like to flirt with him through the fence though, so maybe there’s a reason why they all have a crush on him.

Now seems like we will need to try to match him up with another gelding instead of a mare.

How common is this?