Very reasonable questions. Your first question -- "Why is it creepy to stare at a beautiful woman if it is not creepy to stare at a sunset?" -- doesn't take into account that a sunset is inanimate. You don't have to be concerned with how a sunset will feel about being scrutinized, or what the sunset will think if you tell it you think it's beautiful. The sunset doesn't care.

I don't think anyone would tell you that making a comment to your friend about a beautiful woman walking by is wrong, like you would about the sunset, but telling the beautiful woman that you like her cleavage, quite simply, threatening.

I think it is very hard for most men to empathize with how threatening sexual attention actually is for most women, so here is an analogy: Imagine if a gorilla were trying to make it known to you that he thought you would make a nice lady-gorilla. Maybe he's just fucking with you, but maybe he's not, and he's planning to drag you into that alley or follow you until you're alone and make you his lady-gorilla whether you want it or not. Now, not all men who want to compliment nice young ladies are gorillas and certainly not all men on the whole are, but men are bigger and stronger than women, and some men are gorillas.

Now imagine the gorillas have man-suits and you can't tell them apart from nice young gentlemen. As a woman, you are constantly playing a guessing game to try to find the gorillas. Unfortunately, that means if a man says something that might be sexual, it's in the woman's best interest to err on the side of interpreting it as a danger.

As far as work-related situations go, that is actually very tricky because a lot of work environments have strict rules against employee relations. Besides which, it's just a bad idea! Think really, really hard about what you want to accomplish by complimenting a woman you work with about her low-cut or close-fitting garments. Do you just want to tell her she looks nice? Then tell her she looks nice today and be done with it. Do you want to bone her? Then hang out with her outside of work, and keep your sexual compliments off-campus. But think really, really hard about whether it's worth it to pursue that, given that you have to work together.

I can give a couple examples of good and bad attempted compliments/pickups from my own experience.

Negative example: There was a security guard at my workplace that I frequently said hello to, sometimes shared a few lines of conversation with, nothing more. Then one day he told me he wanted me to show him around the city and I should give him my phone number. He was being kind of pushy: I had never offered to show him around, but he was trying to invite himself over to my place, so I told him no.

All that was fine.

But then he kept pestering me. He would not stop. I told him directly: "I am not going to give you my phone number. Stop asking." He kept asking. I told him that we have a company directory and he can look up my email address if he really wants to talk to me. I then sent the resulting email chain to HR and I think he was fired. He certainly stopped working the front desk. I also checked with some other young women who work there and they said he did the same thing to them.

Another mixed positive/negative example from work: I used to ride the company-provided shuttle to and from work and talk with the shuttle drivers (I live closer to campus now). The morning shuttle driver asked if I wanted to bring a group of friends I was in charge of entertaining to a free concert at his other workplace that weekend, so we exchanged numbers.

We didn't go to the concert, but he and I kept in contact (I'd stopped riding the shuttle at that point) and a few weeks later we met up for lunch in the city, which I thought was just a friendly thing since he is old enough to be my father. So far so good. But as soon as I got in his car he started driving me out of the city ... he put his hands on my neck in a strangle-hold jokingly at one point ... and then he started telling me graphically about losing his virginity. While I am trapped in his car. After he has touched me on one of the most vulnerable parts of my body without my invitation. When he had never made any indication that this was supposed to be a "date." Line: crossed. I told him to pull over and let me out of the car. He might have been able to swing a nice mistaken first date if he had not touched my neck and graphically described sex acts to me.

Positive example: I am walking around my neighborhood in Chicago, which is not such a nice neighborhood, and smiling and nodding to everyone, as is my wont. A nice young man says "Hey pretty lady!" I say, "Hey, how you doin'," and keep walking. End of conversation.

Negative example: I got on the bus to ride home. As soon as the doors close behind me, a man says "I'm looking for a sexy white bitch to impregnate!" The bus starts moving. There is no escape.

Positive example: Walking around my neighborhood wearing a nice dress and heels. Comment: "You look great!" End of conversation.

Negative example: Car pulls over to me (I'm on the sidewalk) when I'm walking home. Driver rolls down the window and asks how much it would cost to spend the rest of the night with me.

Positive example: I sit next to a man on the bus and he compliments my big long winter coat, saying he's glad to see a pretty lady taking good care of herself and making sure to keep warm. He strikes up a conversation about god knows what and when I get off at my stop he gives me his business card. (I didn't call him, but that's how you do it.)

Negative example: I walk past a man in a wheelchair with three friends. He says, "Damn, I'd walk again for an ass like that!"

Positive example: Again, walking around and smiling at people, wearing my big ol' winter coat. Man says, "Why you covering up your pretty frame on a day like today?" I tell him it's cold out and keep walking. He laughs and says it's not that cold. End of conversation.

TL;DR Give general, polite, non-sexual compliments, and don't be pushy. Don't say anything to a stranger you wouldn't say (or want said) to your niece. If you want to tell a woman that you work with that you find her cleavage distracting, don't.