Exactly, what CxA proof are you talking about?
I honestly do think that Cloud can romantically be linked to Aerith before the Lifestream scene. But we already know at the time that Cloud was not himself - if Aerith did love Cloud, it was a fake the whole time. Or even more so, a clone of Zack. It doesn't make sense for Aerith and Cloud to be together when a.) Cloud is already living with Tifa b.) Aerith is with Zack in the Lifestream c.) They would pair up Aerith with a fake in the duration of Cloud and Aerith's time together, and d.) Tifa would have to be rejected by Cloud. And we most certainly do not see that. Square Enix never intended to make Tifa's character to cling on to Cloud even though we already 'saw' her being rejected in the Highwind scene. That's just too out of character for Tifa and Cloud.
Aerith plays an important role in Cloud's life, but that doesn't make her romantically involved with him. Unless there's proof of Cloud's thoughts (not Aerith's. Because Lifestream White only has Aerith's thoughts. Therefore, they are not koibitos), Cloud and Aerith are zilch.
We do have proof of Cloud's thoughts. The core of his subconcious, his 'heart' if you will.
Sadly, Aerith features nowhere inside it. It would seem any lingering attachment he may have had while she was alive was insufficient to warrant inclusion on his self of selves, the time when we get to know him.
Cloud possibly loving Aerith is possible, hell, it's part of Disc 1's one two red herring suckerpunch combo, but it does not seem to last beyond her death, or at least not overshadow the fixation on Tifa he has.
CCU- Cloud fell in love with Tifa before going to SOLDIER.
FF7, Lifestream sequence- Cloud's tender memories and a secret wish no one can ever know are his memories of Tifa, and his wish to be noticed BY HER.
FF7- Highwind scene- Cloud says there's so much he wanted to tell Tifa, but could not find the words, and that nothing has changed since he was a child.
Cloud fell in love with Tifa as a child.
He later says nothing has changed since he was a child.
QED.
That's the sort of thing that makes people who seriously suggest Cloud with his head back on together doesn't want to hook up with Tifa and would rather date a dead girl not quite right in my playbook.
So It's not about Koibito. It's not about her being an important woman and the mother of Cloud's family unit. It's not about her seeing Cloud as the father, or her love interest, or even that 'ideal love' quote that's actually a jumble of about five slightly inaccurate quotes like all the rest of the XThost profiles, hell it's not even about the goddamn sex under the Highwind, it's actually not even about the basic fictional commonalities that we the viewers are being shotgunned with in game, movie, and novella, though ALL of that helps.
It is about Cloud's stated intentions and the intent shown by his actions.
And THAT, my good fellows, is why the C/T conclusion is bloody well inescapable. Because for as unforgettable as Aerith is to Cloud (deliberately omitting such for anyone else), it is Tifa that two simple statements shows he is into, and then you pile on everything else as well. There are great heaping mounds of glorious interconnected statements, actions, and other such nifty things that continually pepper you with the idea of C/T. Even if we take in COLW that C and A were lovers, because of the damn rest of the evidence, they aren't any longer, because Cloud is now with Tifa in a romantic sense.
Also, go ancillary proofs.
