
Not true... It happens with straight couples all the time. Ever heard of "friends with benefits" before? Ever heard of the "unrequited love best friends" scenario before? Ever heard of an "open relationship," or a "non-committal relationship" before? The scenarios commonly have one or both people in love with each other, but neither will tell the other, so they tell each other "if there's someone you start to have feelings for/if you find someone else, we can just end it." Granted, it isn't because the relationship itself is taboo, but straight couples do it all the time...
Hell, one of my Exes told me the same thing a month before he dumped me... "There's plenty of people out there that are better for you than me, so if you find someone, just tell me and I'll leave." And he followed it up with the MAIN reason for why straight couples do this (in my opinion): "I don't want to tie you down, or make you feel trapped, in case someone better for you comes along."
-Shadow
always hurts me when one character in a yaoi manga tells the other that even when the other gets married to someone else etc., they'd still be happy for them. none of that would happen if people weren't homophobes.