It's a matter of setting a limit for when the younger part is old enough to set limits for the older part, to avoid that adults take advantage of unsure youngsters. Whether that limit should be set at 16, 17 or 18 is not a question with a simple answer. A 19-year old may be very immature and a 15-year old may be more mature than some 18-year-olds. But the limit has to be set somewhere.
As for the social stigma: I can't speak for Turkey, but in Norway a 40 year old man would be seen as a dirty old pig if he's interested in a 16-year old, even though he may legally have sex with her. The only thing that would change if she was 15 rather than 16 is the legal aspect, and again - the limit has to be set somewhere.