help with new behavior issues!

He's reached social maturity and his behavior is normal.  I wouldn't worry that there is any aggression involved with marking territory and humping.  I never allow my dogs to hump a human, especially a child.  A child could easily be hurt, unintentionally. Children shouldn't ever be at dog parks, IMO. The problem with a dog humping other dogs is that some day they may hump a dog who takes serious exception and decides to correct your dog using the weapons at their disposal.  Not all dogs that visit dog parks are well socialized.  The other problem that can occur is if your dog humps a smaller dog or puppy and hurts or frightens it.  Now that dog may have problems in the future because of your dogs' behavior.  See where I'm going?  Dog parks are full of strange dogs and even stranger people. lol   You have no control of who will be there any given visit.  You say some owners have dogs that misbehave and they think your dog is aggressive. I don't understand that statement but it's enough of a red flag to suggest you skip the dog park.  The runs, walks and training should be all he needs in terms of exercise and mental stimulation.  If you want him to play with other dogs, make some play dates with dogs you know are well balanced and socialized. 

Your dog sounds like a perfectly nice dog and it sounds like you have done great with him.  What's his name?  Pictures would be wonderful. :)

 He is never allowed to hump humans - but I am not sure why he all of a sudden started this and am not sure how to correct him. Just a few times when some of my friends' children have been playing with him and we take our eyes off for two seconds I turn around and he is trying to hump them. Why is he doing this? What is the best way to prevent this? The children he is playing with have large dogs of their own and are very good around him....I am just lost here. I guess what I was worried about at the dog park with the misbehaving dogs is that the dogs that are not well socialized and are unbalanced will hurt my dog that is very well socialized and balanced. But we don't have a fence so it is soo nice to be able to take him to the dog park. I don't run near fast enough for him to get enough exercise out of it. =) ...Sam has only really humped other dogs that are either already playing with him pretty hard or an example would be this larger puppy (much larger than him) that was putting it's mouth over Sam's head - which I know is a big sign of dominace - that is when he starts humping other dogs...so I feel like sometimes they are asking for it?? When Sam makes a small growl for that dog to get off of him and the other clueless dog owners think my dog is being aggressive it bothers me. Young puppies aren't supposed to come to our dog park anyway. I guess my real true problem is how do I work on preventing the humping? I don't not like it and won't tolerate it especially to humans, but don't know how to work on that one....

Put a leash on him and supervise closely when he is around children and the second he starts to hump, make a leash correction and say "no". 

I don't like dog parks and feel the chance of a bad experience far outweighs any benefits.

LSpataro

I guess what I was worried about at the dog park with the misbehaving dogs is that the dogs that are not well socialized and are unbalanced will hurt my dog that is very well socialized and balanced

Your worry is well founded and why I suggested you skip the dog park.

Editing to change my advice on the humping of children.  It's possible that the dog will associate a negative (leash correction) with the child itself rather than the behavior, unless the correction is perfectly timed, which is hard for most people.  Someone else will be along soon to spell out in detail how to deal with this and if not I'll be back, I have to get to work.

LSpataro

but I am not sure why he all of a sudden started this and am not sure how to correct him.

He is doing it because he has reached social maturity.  Humping can be any number of things, from "a dominance display" to sexual frustration, from over-excitement to simple play. 

The way to correct is to distract and re-direct, where possible.  If he persists, or goes back to it, calmly leash him and move him away or exit the park.  He will learn that some behaviours result in the fun being cut short.  He should have a solid "leave" and "come" before running loose in the park anyway, so maybe you could work on that more before taking him again, and even then, could you pick times of day when it is quieter?  If it were me, I would simply not go to the park.

One way to prevent it would be to keep him on a lead.  Or keep him away from dogs/things he likes to hump.  Or keep him away from the park.  Or learn the signs that he is going to hump, and intervene.  Or a combination of these four

With regards to the children...  How old are the children?  The best thing to do is, obviously, NOT "take your eyes off" them and their interactions, even for 2 seconds   Watch, watch, watch.  If he starts doing it, even for a moment - calmly step in and remove him.  Take him out of the room, or crate him for 5 minutes.  Again, the basic message to him is: "When you do that, the fun ends".  If the children are able, you could ask them to move away to a place he can't reach them when he does it, or to issue a direct cue (like "lie down";), but only if they are able and only if you think he will listen and respond first time.  What is normally happening when the humping starts - is it play or rough-housing?  If so, you could get them to play with the dog with a toy (like a ball or frisbee) so that the dog is focusing his attentions on THAT rather than THEM.  I have noticed that humping often happens when the excitement level is ramped up, so stepping in and getting him to calm down and rest during play sessions, and/or encouraging morsedate interactions with children might also alleviate the behaviour.

I would assume with him being from rescue, he is neutered, which normally reduces the likelihood of lots of humping.  If he is intact, then neutering might be a consideration for you, although there is no guarantee that it will stop the behaviour (for example, my 13yr old neutered terrrier still humps on occasion).  There a few explanations for why neutering may not always prevent or stop humping:

1) it is not purely a hormone-driven activity.

2) even if hormones are the main cause, the testicles are not the ONLY source of testosterone in the body.