Firstly a dictionary says what a word currently means.
Meanings change along with how the population uses the words.

Not only that, porneia is no longer translated as fornication, due to it being inaccurate.
Porneia covers a variety of meanings.
So don't be looking into you KJV and pointing out "But it says fornication!"
That's using the definition being analysed as proof :S

So, understandably it's hard for the translators to choose which translation to use where,
and so use "sexual immorality", the closest translation we have.

But what constitutes as sexual immorality?

Well for that it's wise to look at Deuteronomy 22 and Leviticus 18.
Though these are Old Testament quotes, they show God's intent fairly well I think.
Leviticus 18, well, it's detailed to say the least. Not a mention of sex between two unmarried people.
Why?
Well the issue a guy having sex with a virgin is dealt with in Deuteronomy. They must marry.
There's no room for anyone following the rules to sleep around.

And to re-enforce that, just before it I think, it talks about a girl lying about her virginity.
Then upon being married being discovered to be lying, and should be stoned to death for her sin.
What was her sin? Promiscuity. The time is taken to emphasise this, yet the following line about premarital sex has no tone of wickedness about the act. The wickedness comes from not following the rule to then marry.

Instead the one flesh bond that should be shared between two until death do they part, has been spread among more than intended.

So then there's the question of "Ok well clearly promiscuity is a sin, but would it be a sin to have sex before marriage?"
If it were then wouldn't it have been clearly stated in the incredibly detailed Leviticus. Wouldn't there have been punishment in Deuteronomy? God didn't seem to care that it was done, but cared that once they did, they became married.

So, don't think "Oh yeah! I can have sex now!"
Unless you and the guy or girl you're with intend to marry, and understand the grave sin of having sex as a virgin an not marrying. Because if the future spouse is anyone other than those then you have been prmiscuous.
To have sex is a life long bond as strong as marriage to be with that person, after all, it is supposed to result in marriage.

And to finish off, to all you who think that people are trying to weasel their way out of the "difficult route of self-control",
with the excuse that "It just is."

Isaiah 19:13
Matthew 15:9

Don't condemn what God clearly didn't condemn.
He was precise about promiscuity being a grave sin, but also in the importance of the one flesh bond and that when it's created you must be with that person until death.