You can decide for yourself when something is spiritually bad by these criteria:
- It causes doubt in your heart
- You do not want the people you respect the most to know you are doing it
- When you feel very spiritual (for example if it is Ramadan and you have been reading a lot of Quran) you want to avoid that thing.
The ideal Muslim is the one whose private life and public life are the same so that there is nothing secret in their lives that they would not want others to find out about.
As for your specific question, if the anime has erotic elements, then that is forbidden to watch (as discussed here).
If it is not erotic, homosexual relationships are still obscene (fāḥish) and the Quran says:
and do not come near obscenities, whether outward or inward... (from the Quran, verse 6:151)
Watching a show that celebrates homosexuality is a way of “going near obscenities” which the above verse strongly discourages. Therefore the admirable and pious thing to do is to avoid such shows.