If both anime & manga have the same ending, then I can guess this somehow ended in an unexpected manner. About death theories, space, aliens, etc. I'm not too sure where it was going at. BUT even if he was someone who might not have been a human being or on the verge of death, he is what he is. He is Sakamoto and his own existence can't be denied till he passes away. I believe he's livin' as usual with his friends and people who care about him peacefully with a perfect - person attitude yet with that confidence and determination to live. He never went too overboard either or caused major trouble, just leave it be. It's not essential for him to marry, he can carry on with his normal days. Btw, he really looks like an office guy.