Due to the concurrency of church and state at the times these things were determined, age of consent for sex is often tied to age of consent for marriage which, back in the day, was tied to the act of consumation. In many respects that is still the determining factor for the age of consent even in are largely secular societies.
Pornography, a relatively latter development was assessed for age restriction without such considerations, as was homosexual sexual activity. So on it's own merits it was judged appropriate for people age 18 and over, similar to driving being assessed for people age 17 and over. I suspect the arguments for each have been lost in the mists of time but were deemed sound at the time and no one has provided a sufficiently compelling argument to eitehr raise or lower them.
From what I can remember, the general compelling argument against lower the age for pornography is that, like movie ratings, if you can watch it, you can make it and that introduces a whole raft of moral and legal issues that would need to be resolved first. At the end of the day, without a strong public interest do to so, going through the efforts to make the changes or otherwise deal with the unexpected consequences of trying to do so simply isn't worth the efforts. And because this would be a legislative issue, you can guarantee it would be political suicide to do so. No shortage of political careers were curtailled in the long effort to afford LGBTs sexual equality. Nobody is going to put their career on the line for mucky ladies on the internet who aren't happy.