Is it usually wrong for two 13-year-olds to have sex with each other? What about a 16-year-old and a 13-year-old? 20 and 13?
In general I don't believe in a moral system where the central criteria of whether something is right or wrong gets decided by a straight rule.
What I do believe is wrong is when 16-year-old or a 20-year-old projects power in order to make the 13-year-old decide to have sex with them. Additionally I see responsibility to act afterwards in a way that the experience creates no emotional wounds or other damage.
Pregnancy would be damage because even if the 13-year-old gets an abortion getting a child killed inside himself leaves some emotional trauma. That mean...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.