If you don't understand why people are worried about playing underage characters in sexual situations then you have not been paying attention to current events. There is a scary climate of monitoring and fear developing.

Vore is one thing. As long as there is no sex involved you could say with the straight face that you were just reenacting the older, less family friendly version of Little Red Riding Hood. Hopefully the subject of vore being a sexual fetish would not come up. Many vore RPers won't play with you if you are playing an underage character, but some will.

But you were on a zoo forum where all the play is going to be sexual. Using underage characters in explicitly sexual situations, even fantasy ones, may be legal but it is legally questionable enough that most role playing communities won't allow it for fear of legal prosecution.

Whatever your opinion on the subject, underage play in sexual situations is simply not something most people or most roleplaying areas will tolerate. They have a well founded fear of repercussions and don't want to meet the Why Don't You Have A Seat Over Here guy.