I seriously doubt any jury would convict you, or your wife, for videotaping each other having married sex. The whole point of criminalizing child porn is that it exploits minors for the sexual gratification of others. The “exploited” in this case is your wife. If she videotapes herself masturbating, clearly she’s not exploiting anyone.
If a sixteen-year old posts a Quicktime MPEG of himself jerking off to some BBS, he’s not going to be convicted of child porn because the very age which makes his naked image illegal also protects him from responsibility. The government can’t have it both ways and declare that someone is simultaneously “too young to consent” and “old enough be liable.” You can’t be the victim of your own crime.
Also, child porn itself has fuzzy definitions. A Robert Mapplethorpe exhibit, your mom’s photo album, a nudist colony’s brochure, and “RussianLolitas.com” all contain naked pictures of pre-teens. They’re not all equally prosecutable because intent counts. As it should. The tricky part is if, say, someone takes a photo from a nudist colony brochure and then uploads it to a sleazy website. Then it’s porn. But is the person who took the photo, or the nudist colony that authorized the brochure, or the graphics company that printed it guilty of child porn?
This is why we have grand juries before actual trials. If a judge thinks the case is ridiculous, as he would if you were caught with photos/videos of your legal wife, it will never be prosecuted. If all laws were cut and dry, lawyers wouldn’t hav to go to graduate school for 4 years.
Also, assuming your wife is post-pubescent, even someone else who steals a copy of the video could mount a legal defense saying “I didn’t know she was 16. she looks 18 to me.” If a jury is unconvinced that he knew otherwise beyond a reasonable doubt, then he wouldn’t be convicted. The porn companies that produced Tracy Lords’ movies were not arrested for child porn. The porn actors who had sex with her on film were not prosecuted for having sex acts with a 15-year old girl. Because they could reasonably argue that they didn’t know.
Now if you two made a website and charged people for viewing vids of your wife masturbating, the moment you turned 18 you’d probably be in some trouble for trafficking the images of your “exploited wife”. But so long as she testified on your behalf you’d probably still avoid conviction.
However, the publicity of a trial would then make it that much easier to prosecute other people who downloaded and possessed the images, because they “know” that they are illegal. Just as it would be easy to prosecute somone who tried to sell Tracy Lords movies today.