The reason his wedding is secret is because he afraid someone will hurt her because she is with him. He went to a large cave and did it there.

The wedding song he sings to her.

The cave

He wasn't so sure about but Sagewing seemed so happy he agreed.

The rings

They did it as cats to avoid suspicion because he was a paranoid boy. He was worried cats might hurt her because of him.

Sagewing dress

Jaggedhawk tuxedo 

He literally could not breath in that XD

As a human

Sagewing 

Jagged you know

Fishkit dress

He had a ton of snakes slither around the place, throwing flowers as Pipi turned into fog and wrapped around them. It was magical