So, we never had a problem with the male cats spraying inside until my mother brought home an adult male cat [Spock] and ten months later another adult male cat [Meeko].
Spock sprays outside a lot, which we don't care about because he's outside, but ever since he showed up my cat Murdoch has been marking his territory inside.
Murdoch is kind of a mafia-kitty. He's 'top-cat' and intimidates the males who were adopted as adults. He doesn't have problems with the cats he grew-up with or the ones he saw as kittens. However, he cuffs Spock and Meeko. They don't get into real terrible cat fights--not like they used to--but there is some shuffacuffanin' still.
All the cats are fixed so there's no smell. Rarely do we actually smell cat. I keep an eye on Murdoch and if I catch him about ready to spray I grab him and put him in the tub [he doesn't like litter boxes, and he'll go pee right down the drain] but he'll still spray. Like a few minutes ago, he found where Spock had been sleeping in the bathroom and he sprayed on the towels.
Meeko just recently has started-up the habit too.
We try to keep them outside, but we have coyotes where we live and I lost three of my cats to them. Being that I do love my cats, I try to keep them safe inside, especially since it seems the coyotes come closer to our house during spring and fall.
I'll be keeping Murdoch inside a lot for the next few months and I'm just tired of him getting 'pissed' off. I can normally catch and clean it, but not always and I don't want the house to have cat pee on the walls and baseboards even if I can't smell it.