We don't say these statements are (necessarily) true. We say, what *if* they're true?
MSs make the canon het character fall in love with you. Yaoi makes the canon het character fall in love with his best friend. Why is the former any more heinous than the latter?
Otherwise both make the author and the character engage in very unlikely conversations indeed.
Answer in all honesty. Would you post your favourite fantasy if you thought 75% of fandom would ridicule you or flame you for doing it? Everybody hates Mary Sues except the people who write them. So people who write them (and post them, even more) must necessarily have the courage of their convictions.
"If A saves B's butt, then B must save A's butt. If A screws B's butt, then B must screw A's butt." This has a geometric balance to it, but if geometric balance is sexy then people would orgasm at the sight of Cheop's pyramid. The absolute equality approach smacks of accounting, the most unsexy occupation in the world. 'The active-passive books must balance at the end of the day or the heavens will fall!' You think?
It's a first cousin to that Golden Oldie of erotica, masochism. And these days it has the thrill of the forbidden, like kid sex used to.
'Men *do not* get pregnant! Men *cannot* get pregnant! Would you cut it out with the male pregnancy stories already?!' But equally, 'Men *do not* have wings! Men *cannot* have wings! Would you cut it out with the winged bishounen already?!' Bye-bye, CLAMP, Yuki Kaori, Motoni Modoru, Minekura Kazuya, Higuri You, Matendou Sonata, Deimos no Hana-yome, and a host of manga and OCs too numerous to count.
Do you say, 'I'm going out for small pieces of raw fish placed atop mounds of vinegared rice' or do you say 'I'm going out for sushi'? Do the Japanese sleep on 'bedding' or do they sleep on 'futons'? Why use an inaccurate, prolix or colourless English phrase when you have a perfectly good Japanese term to hand? There is no exact equivalent in English for nii-san, aniki, nee-san, ojisan and obasan (especially when used about strangers), oyaji, kawaiii!!, senpai, kouhai, -chan, -sama, 90% of the uses of -san, and 90% of the uses of sensei. You cannot express in English the nuances of, and the huge differences between, atashi, watakushi, boku, ore, ora, and ore-sama, which all turn into 'I'; between kimi and omae and anata, which all become 'you'; between gomen, warii, and sumimasen, which are all flattened into 'sorry.' So you have to use the Japanese for these and a host of other words.
People have been screaming about the use of newfangled 'foreign' words in English since Shakespeare, and quite rightly. These fashionable and 'sophisticated' words are the Trojan horse that get foreign ideas into a closed and culture-bound society. Shallow? Of course. If they came clothed in Serious Purpose, no-one would touch them with a bargepole. Too much like school. (And as for inaccurate- really, if you can talk about a 'chaise *lounge*' why can't you talk about your 'koi'?)
This one is self-evident.
'In my young days when I was giddy and enthusiastic and carried away by the newly discovered joy of fandom I a) used fangirl Japanese b) wrote Mary Sues c) killed off the canon girlfriend so I could write yaoi d) had my willowy bishounen weep in each others' arms e) wrote prose so purple it was mulberry. It was great fun at the time but now I'm ashamed of it. Therefore *you* must not have fun doing any of these things or I'll pillory you before the face of fandom.' I hope we can all see the problem with this one?