Kaworu wasn’t gay. His ambiguity was meant to reflect the nature of angels since he was one - he loved Shinji with a divine type of love, not a sexual or romantic one. Agape vs Porneia/Eros.
The reason why he acted “gay” is because that’s what Shinji needed from him - that unconditional love that he didn’t receive from his father. If Shinji had been an alpha male, Kaworu would have presented himself very differently.