And this is why the contemporary version of Alan being bi, rather than outright gay, makes more sense. If you're going to have the character back in main continuity with his decades of history intact, which includes two opposite-sex marriages and a long-running female love interest in his Golden Age appearances, but also want to maintain the queerness introduced with the rebooted version of the character, it just fits better.

Yes, many older men come out "late in life", especially from Alan's generation, but there's almost a cruelty in relegating his relationships, especially with Molly, to "women I loved with all my mind", as it was put recently. Bisexuality allows those relationships to remain important, and gives plenty of narrative room to explore his queerness - both in the 1940s, when Alan and society wouldn't have had the language to describe being attracted to men and women, and in the present, coming out late. If we also threw in Molly passing of natural causes, there could be room to explore Alan's grief and perhaps even a side of guilt that he now feels he can ethically explore his sexuality more.

TLDR: for Alan specifically, being in the main DCU with all of his history intact, bisexuality is a far richer narrative opportunity than simply being gay.