Because they don't want to piss Jehovah off. Jehovah doesn't like homosexuals, transvestites, wearing cloths made of two different textiles, but turns a blind neutral eye towards molesting children and has his earthly organization not it reported to the police if the law doesn't make it mandatory and in this way not bring any reproach on "Jehovah" 's name/WT organization That's just half of all the bullshit they preach to capture the minds of the gullible who believe in this Deity and join the slave labor force to sell propaganda (for free to keep the corporation profitable) about living forever if you join the cult.
Control over a persons sex life is a way to test the brain washing mind control you have over those the subjects that are submitting to regular sessions. When some one disobeys directives in clothing or sex kick them out until they repent and fall in line. That's a good way to get rid of bad apples and keep the scam up and running. Defectees must be denied access to the faithful or all hell will break loose.