A/B testing is an essential part of on-line marketing and until today, I had never thought about the "ethics" of giving two messages to compare results.
However, it seems that there may a question to be answered here: how far can you go before your testing becomes ethically grey?
Fortunately, I think "the market" looks after the answer; when Facebook's privacy and testing policies became grey, users complained and Facebook responded by implementing more user-centric privacy options.
It would appear that the same will be true for OK Cupid. They have been testing false positives in relationship matching and users are unimpressed.
I think it likely they'll stop this type of testing reasonably soon or their business will suffer.
In other words, Ok Cupid potentially schlepped unwitting Republicans and Democrats off on a date with each other, which sounds like the premise for a reality show.