If you didn't watch the anime, SPOILERS AHEAD!

Shinji's sexuality and his relationship with Kaworu has been reason for debate since... well, since Evangelion released decades ago. While some people see Shinji as a straight teenager with a lot of issues, confused and craving for any kind of affection, some (me included) disagree with the "straight" part. So, what's the deal then?

If you watched Eva, it's needless to say that the anime deeply explores the characters psyche, their reasons to be who they are, and the way they cope with their trauma, to the point where giant robots fighting is not even directly the focus of it. We have a lot of interesting, multi-dimensional characters full of strong points and faults:

• Rei, the girl who doesn't see herself as an individual and just obeys without even considering talking back to her superiors or father figure
• Asuka, the extremely insecure girl who chooses to be the best at what she does so someone can validate and praise her
• Shinji, who is exactly like Asuka but doesn't have (or doesn't FEEL he has) the strength to take an active approach and just obeys orders while whining about not asking to be in that position, wanting some kind of love and validation with nothing in return

With nothing in return. THIS is where Kaworu comes in.

Throughout the show, it's undeniable how Shinji slowly, VERY slowly but surely overcomes his traumas and insecurities and becomes a healthier, better version of himself. One of his worst traits was how passive and accepting (even if reluctant) he was: I'm not gonna ride the Eva, unless someone orders me to. I'm not gonna do this, unless someome tells me to. He (understandanly) was suffering and hating the fact that he never asked to be the savior of humanity, while actively ignoring that no one there asked for it as well, never being praised for doing something so dangerous. Except... by Kaworu. Appearing out of nowhere, he praised and admired Shinji since the beginning, showing even some kind of romantic interest, never asking for anything in return. And this is the catch.

The main theme of Evangelion is how human relations are hard, how individuality is painful, terrorizing even, being represented by something called AT Field (short for Absolute Terror Field). Our individuality is what makes us suffer, what makes us cry, what makes us miserable... but also what makes us capable of improving, capable of feeling happiness. And just like how individuality is a 2-way-road, so are relationships. Just like the AT Field, Kaworu also represents something: he is what Shinji always wished for, the very embodyment of Shinji's most deepest desire of a 1-way-relationship where he receives love and affection and gives nothing in return. But by accepting Kaworu, Shinji would be denying himself the chance to improve and to live human relationships for what they are. When he kills Kaworu, what he's killing is that selfish part of his mind that wants to be the center of the world.
I think it's very clever that Kaworu is, of all the things he could've been, an angel. An alien, incomprehensible force that even if do cares about Shinji and wants his well-being, will end up destroying everyone if not dealt with. When Shinji grieves his death, he grieves that dream of an easy relationship, the possible reality that he loved so much. It is, in fact, painful to improve as a person, but as we see in the show, ends up being the best you can do to yourself and to others as well.

But was that romantic love that Shinji felt? Honestly, I don't know. Even Shinji doesn't know. Shinji is a teenager trying to find his place in the world and trying to discover himself, and of course this also means his sexuality. While Kaworu is a representation of something, he's also a living character, one that Shinji did love as more than a friend, more than a lover even. So is Shinji gay? I don't think so. Straight? No way. Bi maybe? Who knows. Not even Shinji himself, and that's part of the point here. Shinji and Kaworu's relationship is not directly about his sexuality, but about the things you have to let go in order to experience real relationships and be a full human being.

Kaworu is gay tho.