Sponge should detect whether it’s running headless, but regardless, you tried running the mod file instead of forge.
The following message would have appeared:
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Oops! You attempted to run the Sponge Forge Mod directly!
This jar file is a mod for Minecraft Forge. You can run this file as a Forge
mod by installing Minecraft Forge and simply dropping the jar file into the
Forge "mods" directory. You can get Minecraft Forge from:
http://files.minecraftforge.net/
Once you have downloaded and installed Forge, you can load Sponge as a mod.
If you need more help, you should check out our forums:
http://forums.spongepowered.org/
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