Is it normal that i may not enter a home unless directly invited in?

It's fine, and polite! This reminds me of a lady in our neighborhood. I wish people I know would do the same! I know this is long, but you have to read this!!

We moved into our house last year and a lady from the church came to introduce herself. It was New Years day, we were up until 8AM partying. I don't drink, but we had some friends over that love to drink. It was roudy! There were beer bottles and soda cans everywhere pizza boxes on the floor smashed and stepped on and even a twister mat. My friends wanted to play drunk twister LOL! We have deer and moose anlters on display on a couple tables, but we threw them in the corner for the night so we had places to put food and stuff. Not only that we hadn't cleaned up Christmas yet. There's a dying tree in the corner. We had a snowmobile themed tree display and all of our snowmobile gear had been played with the night before. There were helmets, gloves and jackets on the couches and chairs. I got a treadmill for Christmas and there was a huge box laying on our dinning room table and the treadmill in the doorway, not yet in the workout room. The topper? We live in a older house with no garage, but a huge kitchen. We both have sport bikes and I told my husband he can store the bikes in the kitchen for the winter. So, there was his CBR and my GSXR in the kitchen lol! It looked like the ultimate bachelor pad bomb went off in our house! Before I went to bed a did a load of laundry and hung all the shirts in the doorways. My bras and underwear were hanging from every doorknob in the house. While the laundry was going I cleaned out my purse. There were tampons and pads sitting on the coffee table. Just walking in the front door, you can see everything including the bikes in the kitchen. Sometime in the partying, I came down with the flu on top of all this. When we finally went to bed, this little church lady came to our door. My husband got out of bed to see who it was. He only poked his head out because the house was insane and he had no idea who this lady was. This lady was really wanting to come in to meet me. She was acting weird almost 'obsessy' as he put it. He said no, no, no. Come back later, all that jazz. He told her I just laid down, not feeling good and to hold on. He shut the door to come talk to me. When I have the flu, the only thing that makes me feel good is to be naked in bed because the sheets are cold. I was laying in bed naked came in. I said, "No way!" Tell her to come back later. He went back to the door and told her I'm not in any condition right now and to come back in the evening that maybe I'll feel better. She kept pushing and pushing and said weird stuff like, "I just want to meet her and see her face. I just want to see her face!" My husband fought and fought and told her to wait another minute while he comes and talks to me. I heard him shut the door leaving her on the porch again. He came in to tell me how weird she's being and in mid sentence we thought we heard the door open, at that exact point I felt my mouth water and face sweat. I was ready to spew. I blew past my husband out the bedroom running down the hall naked and there was that lady making her way through my house towards the bedroom! She saw more than my face she saw me naked! She scared the crap out of me! It stopped me in my tracks and then she got an extra treat and saw me throw up pizza everywhere!
WTF!! That lady just let herself in!! My husband pushed past me and grabbed the lady by the shoulders and escorted her through the garbage and out to the front door. The lady kept saying, "I just wanted to meet her." All the while he said she was checking our our messy house. But can you imagine being that lady, walking in to a strangers house...antlers tossed in the corners, motorcycles in the kitchen, Christmas paper tossed everywhere, underwear hanging in your face, dead tree, snowmobile helmets on the couches?! She probably thought I was severely hung over with all the beer bottles (great first impression that never was). Then she gets a face full of bouncing running titties down the hall topped with a puke fetish showcase! At least I got waxed a couple days before! Good hell!!
I never saw her again. They sent someone else from the church and politely offered help if we need help around the house! LOL!! Like we're a welfare case! Of course now they don't come around and my house is now remodled and looking beautiful LOL! What ever! I bet she was more embarassed than I was...so, the moral of the story...NEVER walk into someones house unless invited. It's more for your sake, not theirs.

its the same for buffys enemies they had to wait till clueless mom asked them in

It's fine. It's good manners, but if you insist on standing outside they'll just think you're weird.

Just read the situation more. Same thing with wearing shoes inside. Here it is rude to wear shoes while inside, but in certain situations it is allowed or even encouraged (like in a party, where e.g. women often wear shoes that are not so easily taken off). People would just check if others are wearing shoes or not and act accordingly themselves (reading the situation).

this sounds like the movie Let the Right One In. Really good movie btw.

True story, once i was stuck without a lift in a small town while drunk...so i opened a randoms front door, walked into their house and used the phone.

But why? You already know you're going to come in and that they're expecting you.