Well not every law is their for your gain. Right now you can basically have any sexual fetish you want. Passed out girls, BDSM torture, Race Play anything you want you have full legal rights to enjoy it, but someone who likes 18 year old girls has to live in fear that 1 wrong image means a minimum of 15 years in person.
Its essentially an issues of equality, allowing these people to enjoy their sexual fantasy just the same way anyone else with a fetish can...without fear of going to jail
Just like with most issues of equality the people giving the equality dont benefit, but its a huge gain for the people that receive it
also the age of consent for most of the world is 16 or younger (including europe) So just because you view someone who is interested in a 16 year old as a predator doesnt mean that is the common assumption, even in predominately christian nations