I can certainly understand your negatives, but perhaps I can provide a few counterpoints here.

--I didn't feel it was cautionary at all, merely based on how the creature was portrayed. There were some themes of intimacy and trust regarding sex to be sure, but I'm not sure that it was saying that hookups are bad. The It follows you forever, until you either die or choose to pass it on. This becomes a morality issue, a sort of test to the characters, well, character. The girl in the opening didn't choose to pass it on. Jeff/Hugh did by deception, but then told Jay about it. Jay gave it to Greg with his explicit knowledge, but he didn't believe her. Jay presumably gave it to the men on the boat who were killed when they didn't know. Jay finally gave it to Paul, so they could be there for each other until the bitter end.

I think the sex was more of a tool to make it harder to transmit. It really hammers home the morality of having this curse. You can't pass it on accidentally (if you know about it), you have try fairly hard.

--Personal opinion : those are fun things to speculate about. They weren't particularly relevant to the plot of the film, and perhaps would have detracted from the terror of the unknown.

--How are they going to find out about It? Through a fun library montage? Some creepy old lady? It is invisible to almost all human beings and works very hard to kill the ones that can. Jay's friends didn't even believe her until relatively long into the film. I think the film would have been a lot worse if this had become the focus.

--Jay certainly changes a lot through the film. While she's scared the entire time, she goes through the whole internal morality battle I mentioned before. The fact that she ends up with Paul speaks a lot to how she's changed, both that they're a couple, and that she's willing to pass It to someone else, especially someone she loves.

The point of It was the inevitability. Even if you pass it on, It's still there. You can't get rid of it, you can only watch, and run, and wait. A loophole would have spoiled all the dread of the monster.

--The girl in the opening gave up. It chased her for too long and she didn't want someone else to have to go through the nightmare that she was. She was both stronger and weaker than Jay, and when she couldn't live like that anymore, she went somewhere nice, sat down, and waited for what may come.

Jeff/Hugh was necessary. How else was the monster going to be passed along? They developed him as much as they needed to, and brought him back when it was relevant. Not every character in a story can be a central figure.

8/10, great film, we'll see how it holds up over time.