Eco-consciousness has actually been an issue in many of my relationships. I'm definitely a lot more environmentally aware than most. Not only do I not drive (I never even got my license - I bicycle, take public transit, and even hitchhike when the situation calls for it), but I also make it a point of doing things like carrying stainless steel cutlery with me so I won't have to use throwaway plastic sporks. Several months ago, I went on a (vegan, for me!) pizza date with a guy who looked at me as if I were an alien when I pulled a reusable take-out container from my bag to put the leftovers in. "Do you carry that with you ALL THE TIME?" he asked, looking horrified. "Yeah, usually!" I admitted. It was true! I eat out often, so I like to have a way of transporting leftovers that doesn't involve styrofoam or unnecessary leakage. I knew that I'd sealed my fate, though. For him, I'd crossed the line. I guess it's good for these things to come out in the wash earlier rather than later.
Both of my current serious partners support my eco-nerdery, though they're far less vigilant about it than I am. My boyfriend never brings bags with him when we go grocery shopping, for instance, whereas I always have both big canvas ones and small plastic ones (for the produce). An ex of mine grated on me because he professed to care about the environment but didn't own a bicycle and would drive his car extremely short distances - 3 miles or so - that he felt were too long to walk. He made a lot of excuses for why he didn't have a bicycle but in the end I think it just came down to laziness - he lived in a subtropical climate in a city with great roads and conscientious drivers. I don't mind folks driving when it comes to a roadtrip, but for just tooling around town I feel really wasteful, and I've noticed that it puts people in bad moods when they have to deal with traffic, parking, etc. in the city center.
When I lived in the wine country, I used to commute to work on bicycle, 18 miles each way. Perhaps that's "extreme", but I really enjoyed the exercise - it's a nice stand-in for a pricey gym membership - and the scenery.