Does merely watching some Aikido practitioners prostrate to each other make me sinful?

It is not permissible to watch sports that include pagan rituals, as it is forbidden to watch evil unless it is with the intention of denouncing it. Watching them implies satisfaction with and approval of evil, and it may have dire consequences. Therefore, caution is necessary.

For further benefit, one can refer to fatwas related to the rules for watching sports, the ruling on wrestling and boxing, and the ruling on yoga.