My sense is that somebody who seriously espouses One True Pairing believes that X and Y are meant for each other and no-one else and would refuse to read about, say, how Galadriel fell in love with her husband, since she's obviously meant to be with Gimli and any other suggestion denigrates their pure, true love.

My own attitude towards the OTP concept is better expressed by the icon of franzeska's which can be viewed, for example, here (http://www.livejournal.com/~franzeska/42927.html).