For me harmful or not harmful just is so not relevant as a determining factor.

For me it all comes down to holiness.

One can have the most loving, consensual, apparently beneficial relationship ever but if it's unholy that makes it simply wrong.

Take a straight couple who are doing everything right by each other and are madly in love... but one of them is already married to someone else so that the love they are enjoying is adulterous.... this would be unholy and thus wrong.

The first question that must be answered regarding man/boy sexing in my thinking is: "Is It Holy". If the answer to that is "no" then nothing else matters - it's just not on.

Blessings
Cat.