Friendship means more to the vampires than romance or sex.
The writers have made it abundantly clear that it is not romance, monogamy, sex or even marriage that solidifies a relationship for the vampires. It's friendship.
Laszlo is Nadja's best friend. She was jealous when Laszlo declared Sean as his best friend.
Nandor gave Guillermo his friendship which is a gift he does not give to just anyone. It's special.
Maybe if The Guide was only interested in sex then the vampires would be more receptive to her. Instead she wants their friendship which is why they are so distant from her. Guillermo has earned their friendship whilst The Guide manipulates and schemes. Friendship is where the vampires draw the line because it's the most precious thing in their eternal existence.
We have to remember that the vampires are hundreds of years old. Most likely, sex would lose its charm when you've experienced it thousands of times and romance may lose its appeal when its doomed to end over and over again when one human lover dies after the other.
Whereas true friendship, and even living under the same roof as one another, is meaningful. So I don't think it's that the writers are adamant against Nandermo being romantic, I think we may just be forgetting that eternal life plays a major role here.
Even Nadja was bored of her marriage to Laszlo in season 1. Their romance has its ups and downs but their foundation of friendship is what keeps them happily together forever.
It's not queer baiting. It's not ship baiting. It's not even the writers toying with the audience. It's about creating a show that is oddly deeper than what it appears on the surface. I think writers who lacked integrity would have made Nandermo canon by now because that's what we want.
My point is Guillermo and Nandor could be in a romantic relationship, have sex and hell, even get married. What I'm saying is that those things may mean something to us and even Guillermo because we're human. However, those things may not mean the same to Nandor.
Nandor has probably had thousands of lovers.... and to this day, he still loves John.... his horse, his companion, his best friend. Do you see what I'm getting at here?
Guillermo being Nandor's best friend means that he is the most significant, cherished and loved person in his eternal life. That's what "best friend" means to Nandor. The friendship is what makes Nandermo canon and we have that now.
A kiss or sex or a love confession are the cherry on top but Nandermo went canon in the s5 finale. Maybe that's just me.