I think a sin is an act resulting from a failure to love your neighbor as yourself.

The other half of what Jesus called 'all the law and the prophets' is 'Love the lord your God', but I don't think lapses of faith are sins. I think they are just lapses of faith. Faith is a tall order, and I think it is only our own loss if we are not able to rise so high. God will wait for us to know we need to come back.

What you do in your fantasy life is not a sin, unless it motivates you to do an actual act to an actual other person that falls short of their due in love and respect.

There is no sinfulness in states of being. Sexuality itself cannot make you sinful. You are not in a state of sin if you have BL thoughts.

"For my yoke is easy, and my burden is light."

With Christ, you are not in the grey. The sky is blue.