The greenest sex would require no lube at all, but let's be realistic: sometimes nature can be improved upon. Still, you might not want to grab the lube you're already using.
Silicone-based lubes are essentially nontoxic and as slippery as it gets, but they're still made from factory-produced chemicals. Oil-based lubes are sometimes petroleum-based, which degrades the environment and latex condoms and diaphragms. Yikes!
Your best bet for safe sex and a better environment is a water-based lube without glycerin, petroleum or other parabens. Firefly, Sliquid H20 and Good Clean Love are all good options. Babelube Natural fits the bill and contains organic ingredients, like aloe and carrageenan (which, some studies show, helps prevent HPV transmission).