The most obvious reason why it is stupid is simply because it is unnecessary. What percentage of 16 year old girls will have sex with a much older man? The vast majority of 16 year old girls are already naturally repulsed by the idea, and could never be persuaded into it, so a law banning older guys from having sex with 16 year old girls is totally unnecessary.
Second, 16 is during a woman's peak, and it is ridiculous to set the age of consent to be during a woman's peak. If you doubt that 16 is during a woman's peak, then let my ask you this: Is a woman more attractive at age 16 or age 50?
Third, every time you set a hard limit, you have to recognize that EVERYONE who violates it is guilty of a crime. People often advocate for these limits with sweet, innocent 16 year old relative in mind, while neglecting the 16 year old thuggess, who is fully developed and has been a slut for years.
Fourth, it is better for teen girls to have sex with older guys any way, because an older guy is more likely to have a job and provide for children, if she happens to get pregnant. If a woman gets pregnant by a teenage boy, he will most likely end up being a deadbeat dad.
Keeping these things in mind, a more reasonable age of consent would be 10. Not because 10 is a good age to have sex (and this is the distinction that most morons here cannot understand), but because this is the age where enforcing the age of consent would do society a benefit.