Thanks for the question! Very interesting :) also - what follows is mostly my own interpretation based on canon as I don’t think there is a lot of textual evidence on these things…
First of all, I don’t believe that Dean explicitly knows about his feelings for Cas. I believe Dean is aware that his relationship to Cas is different to the one he has with Sam - but he hasn’t gotten as far as putting a name to it. I think it’s very difficult for Dean to differentiate between concepts such as ‘love’, ‘want’ and ‘need’. Mostly due to the neglect he suffered as a child and the skewed idea of ‘love’ that resulted of that (I’ve actually talked about this in a lot more detail here in case you’re interested).
What happened in 8x18 and the conversation Dean refused to have with Sam wasn’t entirely due to Dean’s romantic feelings for Cas, I don’t think so. What Cas did left Dean in a very vulnerable place because it hit him precisely where it would hurt him the most: he basically laid himself bare in that crypt. He told one of his best friends he needed him and not to leave him… which goes against everything Dean Winchester usually does (Dean provides help and direction, he doesn’t ask for it… unless he’s in a very dark place). In Dean’s world, ‘need’ might as well be called ‘love’ (in any sense of the word). And despite all that, despite how defenseless Dean had felt, Cas left again. Cas once again implied he didn’t trust Dean with something - Dean who, only moments before, put himself in a completely vulnerable place in order to get through to Cas. So, when Sam wants to talk about it in 8x18, it’s not about shame or Dean not being ready to talk about his feelings for Cas or anything like that. It’s that Dean’s trust and vulnerability was thrown back at him as though it was nothing (just to be clear: I know that wasn’t Cas’ intention. But that’s what Dean took from it). It’s an incredibly intense emotion Dean is experiencing and reducing it to his romantic feelings for Cas doesn’t do it justice at all I think. On top of all that, Sam is his little brother after all - Dean doesn’t like to show weakness and his identity of The Protector is still in place. What happened between him and Cas goes way beyond feeling weak, Dean was left alone right after he admitted how much he needed Cas in his life. It’s important to understand that Dean needs people to need him in order to feel loved: so him saying he needs someone is HUGE and obviously places him in a very defenseless situation, completely bare of his usual protective layers. There’s no way he would’ve allowed himself to open up to Sam about that. (of course I do believe the intensity of his reaction stems from the fact that he loves Cas… but I don’t think he’s ready to understand that, not yet at least) .
Do I think John was homophobic… hm, well I think it’s complicated. I’m sure there are people out there who’ve thought about this issue a lot more in-depth and I feel a little bit ill-equipped talking about it, but still: I believe he certainly didn’t teach his sons awareness or acted in any way that would suggest any doctrine other than ‘becoming a manly man’. Personally, I don’t think he’d have ‘wasted’ his time on homophobic slurs or the like, but he DID raise his sons in a very heteronormative way (or with internalized homophobia firmly set in place). There obviously was a lot of masculinity in his appearance as a role-model and he put a lot of focus on survival and strength and other military standards with the way he raised his sons and thereby completely disregarding any differing inclinations or desires his sons might have had. In fact, I’m certain he continuously compared traits which society typically associates with femininity to weakness - we see enough evidence of that in the way both Sam and Dean react to pain.
Obviously, this is something that affected Dean especially hard since he wanted to please his dad in any way he possibly could (which, for him, meant idolizing him, copying him, trying to become like him). So I’d say my stance is that John’s parenting had everything to do with Dean’s inability to define and accept his bisexuality which also led to occasional homophobic behavior (the 5x14 cupid scene for example). Not because of outright homophobia on John’s part but because of the normative image of a ‘proper man’ he instilled on Dean (although canon doesn’t really give us much information on that so this is more of a subjective interpretation).
Also, what I think is absolute key in this whole mess is the fact that because of the neglect Dean suffered and his intense fear of not being loved or not being needed enough, he never really developed the tools he’d need to properly identify his feelings with regards to Cas (or any man for that matter) AS WELL AS to fully embrace his occasional inclination toward behavior that stands opposite the ‘manly man’ image (although we keep seeing the occasional evidence that it’s there and it’s a significant part of who Dean is). I think for a long time, his idea of a happy or ‘proper’ ending was exactly what he lost when he was four: a picket-fence house and a fairy-tale marriage.
— -> and since you’ve asked this question a couple of other people, I’ll link to their answers as well which are all very insightful (and I actually found it super fascinating to see how similar and yet different they all turned out): dustydreamsanddirtyscars answer is here and sleepsintheimpala posted hers here!