Franklin is an idea that's been offered before and I don't have any real argument against it. So far he's been seen as hetero-normative (I don't know if the term is a good one). He's had contact with Wendy as well as a girl on Yancy Street where attraction could play a role. Additionally, his feelings about Kitty may also be part of this (but I haven't read FF/Men 3 still locked up in my closed LCS). Hickman also made references to a future girlfriend/crush.
Johnny and Franklin seem to be the only real possibilities of LGBTQ representation of core FF characters. We know Johnny's got a strong straight streak so bisexuality seems to be more reasonable than gay with his character's history. As for Franklin, would any sexual preference be an issue given that he's already an incredibly powerful mutant and creator of universes?
Insisting on one or the other seems like tokenism. Both could work. This is certainly a fun and interesting conversation.
I have to admit, I kind of read some of the Franklin/Galactus stuff as a bit more romantic than it likely was meant to be - likely influenced by him taking Frankie 'Nova' Raye's role from 'The Last Galactus Story' in 'History of the Marvel Universe.'