I don't know if you know this but due to the declining birthrate in Japan, gay marriage is ban from the country. So instead of marrying, gay couples in Japan resort to adopting each other.
If you've read the novel then you'll know that there is a connection between Mana and Allen that's deeper than father and son. Not to mention the fact that Neah is now inside Allen as well.
If course, this is not a Shounen Ai so what we're talking about is purely Fanon. But things like Allen x Kanda is also purely fanon as well. Nevertheless, the ships are sailing.
It's sailing I tell you.