Personally, as a lesbian, i've been propositioned by women like your wife before and for me and most lesbians i know, it's a deal breaker. For most of us, it's difficult because we know there is no long-term partner potential. This is a fling, friends with benefits, or at most a poly relationship where we will be a secondary partner. Personally, i'm not interested in being anyone's "something on the side" and i'm not interested in polyamory, although there's nothing wrong with that.
i guess my point here is that your wife needs to explain what, if any, the expectations for the future might be. Is this a one time thing or is is she hoping for a long-term secondary poly parter? Something in between? Or just see what happens? And she should beware that some women, possibly many, just aren't going to be interested in a relationship without the potential of a long term monogamous relationship down the road. Communication and setting expectations are vital. There is nothing wrong with polyamory and it's a perfectly valid relationship and lifestyle choice. More and more people are finding that it works for them and you may very well encounter women who are on board with that plan. But don't try to push anyone down that path if they aren't 100% comfortable with polyamory and, specifically, the version of poly that you and your wife are interested in. While poly works great for many folks, it's truly not for everyone.