Because it is easy to keep in secret while accussing others of something that really don't matter.
When you are corrupt, thief, not charitable, not honest etc. It is easy to point a finger for another one's mistake and make a big fuss about that, than letting people look at you

Sex is beyond their control, it’s instinctual, tying us to the animal kingdom. It gives us both pleasure and the power to procreate. Many envy that.

Morality police, who have no real morality and if they followed their rules to the letter of the law they would find themselves locked up for their many violations of their own code. Such B.S.
I invited a lesbian couple to a party (live in a conservative area) and they were shocked that I and my now ex had no problem with them being public about their relationship. Why not. If they were in the public, why would I invite people who don't respect my other friends.

Because religions - and the people who spread them - seek absolute control over their subjects. Allowing a freedom from religious influence in one aspect of life - such as sex, leads to a general freedom of thought, which leads to the freedom to ask questions about everything else - which, in turn, weakens the grip over the minds and the lives of the followers...