Neutering stops marking right away in about 60-70% of dogs. In my experience intact males will try to mark 90% of the time whenever they are inside anyone's house but their own. I have yet to bring in a male rescue that has not tried to pee indoors in my house, and then again in his new house, usually the moment he walks in the door which can be really embarrassing. If you leave him intact, you will need to keep him closely watched every time you take him to visit anyone. As you can see from Jen's recent posting on this problem (looking after someone's else's male, who is peeing all over the inside of her house) it can make a male a pretty unwelcome boarding guest.
There are other health and behaviour issues directly linekd to keeping a dog intact. I have gone through these many times, but you will likely start to see a lot of humping and mounting of other dogs (I also have yet to bring in a male rescue that did not try to do this to my own dogs, often causing fights); they will likely be more aggressive, they are more likely to get into fights with other males, to wander if they get out -- they will easily go well over a mile in search of a female in heat. I've noted before that when I did general rescue at the pound, over 75% of all dogs in the pound -- and killed -- were intact males (most often clearly someon's cared-for pet) that often came in after following -- and tormenting -- a female in heat. Intact males also often get prostate problems as they get older -- I have had to have this dealt with in a couple of elderly males and this is NOT the point when you want to hear you vet say that if you do not neuter him at age 8 and with heart problems, he is going to decline.
Health issues are not the main issue I would never home an unneutered dog though -- safety and behaviour and responsibility and weflare reasons are. I do feel some pet owners can manage intact males but in my experience, pet owners are very lax in this regard because they always think their male won't be the one to do the things that intact males tend to do. If you ever allow your male off lead, for example, you will always risk two things -- that he will make a run for it if he scents a female in heat and will be gone forever; and that he will in a moment of your own distraction end up mating with same. The pregnant female dogs that come into the pound (or to ME! needing either a spay when pregnant, which is always a risky and SAD choice to have to make, or requiring months of care) do not get pregnant out of nowhere -- they get pregnant from intact pet male dogs that either are left to wander got out, or had a little rendezvous while the owner walked the dog off lead and could not retrieve their male (or on the lead in a moment of distraction, for that matter). In this sense every intact male is 50% responsible for a litter though it is the owner of the female that will have to deal with it -- sadly, most often by dumping the puppies at the pound.
Consider the case with Ginger: someone else's casual attitude towards their intact male has left one person having to spend 8 weeks caring for three unexpected puppies and left rescue needing to home them, with all the costs this two and a half month event has required in vet care, extra food, time and effort. They costs will not be recuperated and are out of Thelly and my pocket -- and meanwhile no one finds a cav cross puppy any more interesting than any other cross as the lack of interest in the remaining female so far demonstrates.
Intact males cause the biggest headache to rescue and animal welfare, full stop -- they fill the pounds, and can endlessly produce litters all their lives. Like most doing rescue work, with either cats or dogs, I get very frustrated by the attitude that 'there's no need to neuter' -- when all that often translates to for the pet owner is, 'it doesn't matter what he gets up to or if there's an 'accident' as I won't have to deal with the outcome'. No, people like ME will.
Neutering saves a LOT of puppy lives and lets rescue people focus on other serious welfare issues.