At 15, society has decided that young people are not yet mature enough to be making such decisions. As a 20 year old man who IS old enough, he's taking advantage of her youth and inexperience in the opinion of society.
Now, maybe you disagree with the chosen cutoff age, but there has to be SOME arbitrary cutoff somewhere. You'd agree that a 20 year old man and a 1 year old girl is wrong, right? Maybe you think 15 is old enough. I'd say you're wrong, but you're welcome to your opinion and can try to change the mind of your fellow voters. But we're just debating WHERE to draw the line. I hope we both agree there DOES have to be a line somewhere.