If you have ethical issues with the site as proprietary software, you must weigh the costs and benefits of your use.
If you think the benefits of using this site (personal enjoyment; public increase of free software awareness on a large platform; support of well-defined community rules and norms) outweigh the costs (using proprietary software and encouraging others to do so via a network effect), then you should continue using the site. If you don't think the benefits of use outweigh the costs of use, you shouldn't use the site.
By definition, everyone here thinks the benefits outweigh the costs. Of course, the cost-benefit tradeoff of a hypothetical site exactly like this one, but run on free software, would be more favorable to you (and others), but no such site exists. You are free to undertake this monumental task of software engineering and community-building, if you wish. (Certainly, hugely-scoped tasks in the past have not stopped free software development, e.g., in building a free operating system. :)
You are welcome also to petition the owners of Stack Exchange to freely release their software, but the odds of this having a positive outcome seem vanishingly small.