Recently, the situtation has changed slightly. My wife has decided that the family issue is more important to her, and she's willing to give on letting me explore relationships with people from our temple (I'm a member of a very progressive LGBT friendly synagogue). I'm not sure anything is likely to come of it, but it feels good to know she's willing to look at my perspective, and I can give on the other issue. BTW, the other issue was never about dating members of her family (which I've always accepted as a no-no); it was more about the freedom to date people I knew through her family, such as their friends, exes, etc.
From more discussion, I've found that she's really concerned about how the situation looks. She accepts the idea that I'm interested in other people, and she's been okay with me going on dates before, but she's afraid of her personally being "outed" (she's not poly oriented, just supportive). She also feels that other people will look negatively at her, as if my being poly were somehow an indication that she couldn't do any better. It's all wrapped up in her self-esteem issues.