Personally, I don't feel there's anything unethical about it. Sure, you might be breaking your university's honor code, but I think the only ethical conflict there is that you are breaking a code you at least implicitly agreed to, not in the act itself. That is, your ethics or ethical principles in general do not necessarily align with the honor code, and I would say in this case they most certainly don't.
I namely think that the stock standard honor code imposed by universities is a dinosaur that needs to revised, and I think many professors are recognizing that. For example, one of mine mentioned explicitly that he realizes that students use the internet for solutions, and that we should just reference the source when doing so. On one of our homework problems, he also gave us a hint in his office hours and then just added "or just look up a proof on the internet", saying either way is going to be fine ultimately.
In addition, I do not see a difference between looking something up on the internet and consulting a physical book in your library. I assume no one would take issue with the latter, would they? After all, the goal is to learn the material.
Plus, research shows immediate feedback is necessary for learning, and by not asking for help on homework (be it your friends, your professor, or "the internet") you're only hurting yourself. You probably have to at least attempt the problem in earnest to get something out of it, but if you can't get it, you gain nothing by puzzling over it without success. On the other hand, if you do stumble upon a solution on the internet, you might find a new trick or a new way of thinking about it, since you'll have to interpret it on your own. Often, answers here are also given by people with a different background, so you don't get the answer served on a platter, but, instead, you have to really look into it and interpret it so that it fits with your specific background and the tools you are allowed to use.
So, basically, I don't think that there is anything, and I mean anything, unethical about asking for help on SE in and of itself. It just might be against the honor code. And as I mentioned earlier, the only unethical thing then is that you're breaking an agreement, which I think is an obsolete one anyway.