With how the series is going into gender identity and other queer topics, a part of me is thinking that Arsenio is going to come to terms with being non-binary and Sauro has already said he is not attracted to male Arsenio. Overall, my personal 'best' way to go through with this is for Arsenio to be non-binary and Rei to be genderfluid, while Sauro remains a stalwart friend and comrade who over time becomes a more self-actualized person on his own rather than through a relationship, ultimately finding a new love with someone else. Or perhaps remaining single if that's what he prefers.