See, I love, absolutely love Destiel. But that doesn't mean I want it to become canon. Personally, I too think that it would ruin the show. Like you said, a lot of people will stop watching, because not the whole fandom ships Destiel (some Destiel shippers also really need to understand that).
Now, the thing about forcing it on Jensen, I think it's extremly disrespectful, and I can't believe that someone does that, because he has said so many times that he doesn't want to hear about Destiel (people who say that's because he's homophobic, shut your mouth). I don't really think that his daughter will be affected very much if/when she finds out about Destiel. Maybe it's just me, but if someone shipped my dad with someone and drew unappropriate fanart of them, I would just see them as really weird and a tad crazy, but then just forget about it.
"It's about Sam and Dean." I've already said that I don't want Destiel to become canon, but it can in fact become canon without all that stupid romantic shit. At least for me. I'd say a ship turns canon as soon as they have expressed their love for eachother (not just say "I love you" because that can mean like "I love you as my brother" or "I love you as my best friend").
I agree that if it did become canon many fans would probably be very dissapointed. I mean, after all the fanfics we've read... I just don't think the show could compare.
And a last thing; I won't call it sexism. Not everyone is like "omg I ship it because they totally looked at eachother >>that<< way." For example, I ship it because I like their backstory and I like the way the interact with eachother. When it comes to Sam and Dean, it's not the same. I don't know how to explain it, but their relationship is just... Different.
Well, that was all I had to say, so bye.