Interestingly, when I think about the poly people I've dated (some of them couples, some of them one half of a couple) I realize that many of my first meetings with them were with BOTH of them together. Usually this has been in social situations like parties or poly meets, but a few times it's been on a "date" with both of them. It actually went ok (well, exceptionally well with the couple I'm currently dating).
So... as a perspective from the other side of the fence, I wanted to point out that if the date is casual and respectful, it's actually sort of nice to see how a potential partner interacts with their current partner. As a woman, I find it comforting to see their poly-in-action and would rather weed out those who aren't comfortable with it sooner than later. If I started dating a man and didn't get to meet his wife for weeks, and then found out she wasn't quite comfortable with me, that would really suck. :P