With one of my friend's geldings, it's not a problem. He's a laid back, lazy kind of guy. My mare just ignores him basically.
My friend has a new gelding. He's upbeat and keeps up to my mare's fast walk. (So nice for a change not to have to rein my mare back!) However, the last time we went out, my mare was just a total flirt and I was constantly after her to pick up the pace (which is really odd for her), listen to me, and quit stopping to wink. It was sooo annoying. Get this though, before we met up with my friend, my mare was perfectly fine. As soon as we dropped them off, my mare was fine again. Not 5 minutes difference, but immediately! So, I know the cause. What I don't know is how to fix it.
I was after her the entire time. She would listen to me when I demanded, but it was just completely unpleasant. I had no warning before we went out. She didn't appear to be in heat before we left. If I know she's in heat beforehand, I can just ask my friend to take another horse, but that doesn't solve the problem.
(Sorry it's so long. Seems like I tend to get long-winded when I explain things.)