Let's try this again. You asked me why I think it's WRONG for an 18 year old to marry a 50 year old. I said I believe it is WRONG for the SAME reasons why the law says its wrong for a 17 year old and a 50 year old to marry. It's wrong because there is no difference between someone who is 17 years, 11 months, 30 days, 23 hours, 59 minutes old and an 18 year old.
So.... no reason, then?
Look, if you can't articulate the reasoning behind a poorly thought out hyperbolic statement, it's okay to say so. You could have just said "it's no more wrong for an 18 year old to marry a man three times his/her age than it is for a 17 year old," to which I would have responded, "sure, but legally the 17 year old has to get parental consent in some states while the 18 year old is considered to be an adult." Your problem, then, seems to be with the arbitrary age of consent, to which I can only say (along with Joph, many times now) that there has to be SOME demarcation between needing parental guidance and having autonomy, and in the absence of universal objective maturity markers it's going to be somewhat arbitrary, like it or not.
It's a long leap from "I see no difference" to "this is wrong". "This is wrong" is a flat statement of fact, while "I see no difference" is a statement of opinion.