None, how the hell can you not believe in another person's subconscious preferences?
If they don't want something, they don't want it, and if they do, they do. Pretty simple to follow. You do understand that some people are truly incapable of sexual desire and/or attraction, right? Homosexuality is accused of being unnatural when it isn't, and the same applies to asexuality and other low-to-no-attraction sexual identities. Just because it isn't indicative of the majority of humans doesn't mean it's impossible, as it could be psychological or genetic in nature and therefore may change depending on the individual. All sorts of sexualities exist and it's a very complex spectrum of identities. Literally any sort of sexuality is possible - what's there not to believe in?