I have a female colleague who is very touchy-feely with everyone at the office. I tried to not get bothered by it, but it has gone to the point where when I step into the office, it's almost like being in the middle of an adult movie. Recently I saw her sitting on the armrest of a chair a male colleague was sitting in. This annoys me a lot and I don't know what to do about it.
I've been in this situation; a simple quick chat resolved it, and improved our working relationship.
If she's too touchy-feely with you, and you don't like it, your first response should be to tell her. Don't go reporting it to a manager or HR; it'll only make your life and her life more difficult. She probably doesn't mean anything by it, and would be mortified that she's annoyed you. Getting someone else to raise it with her is going to really embarrass her.
Remember that in most places, HR (and reporting HR-type stuff to managers) isn't there to help you. HR is there to protect the company from employees. As soon as you make an official report you and her become a problem to the company.
If she ignores you, you might want to take it up with HR then. If they are going to be helpful to you, their first question is likely to be 'have you asked her to stop?', at which point you can now say 'yes', instead of looking like an you're trying to cause an issue by reporting it instead of talking to her first.
If her behaviour with others is annoying you, you should probably ignore it. It's up to them how she relates to them. If it's really distracting, you're probably better off with a jokey 'get a room, some of us are trying to work', rather than involving HR, but depending on your situation, it may be better to involve your manager.
If you truly feel that her behavior is making the work environment uncomfortable for you, you should bring it up with your manager. Don't specifically ask them to take action, but mention that you feel uncomfortable being around this behavior, and ask your manager what the best solution would be. If they feel the behavior is inappropriate, they'll probably ask your co-worker to tone it down. If they think the behavior is fine, they'll hopefully offer some sort of alternative that will make you feel more comfortable at work, at least.
If you can ignore this behavior and let it slide, though, I would recommend that as your first option. Sitting on a male co-worker's armrest hardly qualifies as adult movie material, and it doesn't seem like others are too bothered by it. If she starts to invade your personal space, or physically touch you, you can take it up with her right then, and escalate if she continues.