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.

Problem solved.