I don't get upset if I see people in other pairings, I just sometimes find it hard to get my head around, I sit there going "But...this is out of character! A is in love with B, not C!", unless the author has gone to enough effort to explain WHY A is in love with C instead of B.

I think that's the problem with terms like "ship" and "OTP", they mean different things to different people. I just use "OTP" to mean "pairing I really like, and like the most for these two characters", but I know other people use it in the narrower sense of "If I see fic involving rival pairings it CRUSHES MY SOUL" etc. I will happily read fic for "rival" pairings (as long as it doesn't bash my OTP) it can be fun to see the characters from a different perspective.