The first time I met my now-wife (24), she said in no uncertain terms she didn't want kids. After two years of dating, kids suddenly became super important to her and panic started to set in about possibly not being able to have them.
On a per-capita basis (not per date, as casual dating may be more common), I'd guess that many (>60%, to put a number on it) of those in the dating market have at least some notion of a 5+ year future in potential dates.
My friends and I have generally looked for partners, not flings. A great long term match can be really worthwhile.
Those who look for flings date more. Most men who date are looking for flings. Thus, it becomes more likely that most who date are looking for flings.
See Jaynes's book "Probability Theory: The Logic of Science"
Most dates don't turn into anything serious, but I doubt most men who bother to date are looking for a "fling", your definition of "date" may just be overbroad.
That's not really typical.