I think we are kidding ourselves if we think we do not need boundareis. We are not super christians, well, I am not. If you do not set boundaries, then you have no measure for good or bad. If the bible said what was acceptable, then there would be no question. That is not the case. You are right though, the bible does tell us to flee from sexual immorality. The problem is, again, our flesh. Our flesh will make us want to throw all morality out the window when we are sexually charged. Our flesh will lead us to make exceptions and subjectively define the bible. We are kidding ourselves if we think we do not need boundaries. Traditional courting couples, and even Joshua Harris had boundaries. It is wise to know what is acceptable and what is not. But the issue here is not if boundaries should exist. The issue is what those boundaries should be.
Since I have lived both sides of the issue, I think my council is wise. Do not be sexual in any way with someone you are not married to. When you are dating, or even engaged, you have no right to be in a persons pants because they do not belong to you, you ought to treat her like your sis.
I heard a good boundary from someone here at Christian forums. He said that you should not do anything that you would be embarrassed to watch your parents do. I almost fully agree with this. I have shared that my girlfriend and I kiss, and we have been able to stay to that boundary almost every time. I would not want to see my parents make out, but that is just me. For some, kissing is not right because it will lead them astray. This is just like how drinking is wrong for some, but not for others when done in obedience to Christ.
Regardless of who you are, you cannot say in good faith that being sexual with someone you are not married to is acceptable before God. So my two cents is that no matter who you are, being sexual with your significant other is NOT acceptable.