IMO it has a lot to do with previous training. If you train the horse to be a well mannered, quiet stallion, when you geld him he will most likely be a well mannered, quiet geldinh. A 'do what I want when I want because I have balls' stallion though? Highly unlikely that it will suddenly quiet down a whole lot once he's gelded, even if he isn't necessarily interested in the mares.
We had a foundation bred Quarter Horse stallion who we kept intact while he was being shown and earning points, then we bred him twice to see if his performance abilities carried over. When we found that his offspring didn't have quite the quality that he had, we made the decision that it was unfair to him to keep him as a stallion and gelded him. He was about 14 at the time. He was a very quiet, well behaved stallion, and that carried right on into him being gelded. He was being turned out in a mixed herd 6 months after being gelded and I never once saw him try to herd the mares or mount them, and he wasn't gelding aggressive. His performance ability actually increased as well, and he was in general a happier guy because he didn't have to be isolated any more. We recently sold him but we've used him many times as an 'uncle gelding' for our colts, to teach them their manners. He was great. The only thing I ever noticed about him was that he was the alpha of the herd, and no one ever really challenged him about it,
Opposite of that was our gelding Noah (r.i.p) who was supposively gelded at 10 months. He had a LOT of studdy characteristics even when we got him off of the track, that had to be worked out. He would attack other geldings when mares were around, and he was VERY moody and tried his best to ignore his handler (which was aweful since he was 17.2hh) when the girls were around. The one time we did try pasturing him with mares, he immediately got a 'girlfriend' and started acting like an idiot any time we took her or him away, and broke multiple fences. He ended up having to be pastured either on his own or with Buddy, because he'd injure the other horses otherwise. He even ripped open our 19 month old gelding's neck once over some hay! I think it had a lot to do with lack of socialization and training early on, and I'm convinced that if someone had worked on it, he wouldn't have had to live his life in solitude.
Everyone in your life is meant to
be in your journey, but not all of
them are meant to stay till the end.