Why seek a definition of porneia? Why not stick with (say) the definitions as given in good dictionaries, or follow what the best of the recent translations offer?
In his book Haller raises some questions which I think are good questions, and in following up some subsequent exchanges re reviews of his book I have found at least one other question, so the seeking of clarity about the definition of porneia includes at least the following issues:
(1) How wide is the scope of porneia when it is used in lists of objectionable/prohibited actions/attitudes (e.g. Jesus speaking about what defiles a person, Matthew 15:18-20, or Paul writing about sins which may be problematic about fellowship with sinful Christians, or are going to prevent entry to the kingdom of God, 1 Corinthians 5:1-6:20)? Specifically of interest in respect to current issues in human sexuality is whether porneia covered both heterosexual illicit sexual intercourse and same sex illicit sexual intercourse.
(2) In the end, is the weight of Scripture against 'sex before marriage' in general, or just in cases where sex between two people does not lead to their marriage? Specifically, does porneia mean 'fornication' (or include 'fornication')? A Malina article in 1972, vigorously disputed by a Jensen article in 1978, argued that porneia did not rule out sex before marriage where marriage later follows.
In other words, while translations which offer 'fornication' or 'sexual immorality' as translations of porneia are entirely laudable measured against the definitions provided by the better dictionaries, the question is whether they are sufficiently accurate for readers today who might (say) rule out more sexual activity than Scripture requires, or (say) misunderstand the historical background of the life situations into which porneia spoke.
So, it is worthwhile taking time to tease out what is going on with the use of this word in Scripture.