There is a difference between enlightened mind and ordinary (deluded) mind.
Ordinary mind see things as ESSENTIALLY different. They differentiate AS REAL for example shaking hand, or sexual discourse. Some ordinary mind see sexual discourse as disgusting, some see sexual discourse as holly, or some see that as just normal activity.
Ordinary mind really classify things as if, some thing is really disgusting from the object side, or something is really holly from the object side, or something is really neutral from the object side.
However, this is illusion. All of them are just the game of the story of your mind.
For enlightened mind, they are able to see the illusion of their mind and realize this illusion.
So for them sexual discource or shaking hand, they are no difference. Sexual discourse is not better than shaking hand, or worse than shaking hand, or as neutral as shaking hand, and vice versa. All of them are just one taste - empty of story.
"once a tantric adept attains the mantra'siddhi or the gnosis-siddhi (jruma-siddhi)"
In buddhist context, we should not interpret it as someone who can show some magic power as equivalent with someone who realized emptiness.
They are a lot of people who behave as tantric practitioner being able to show their magic power, but they don't know what is emptiness. They disguise their power as being holy and do all these evil deeds. In this case, tantric practice actually kill him.
"he is allowed to eat and drink and have sexual relations not just according to the customs of his country and social class but just as he pleases, because he has transcended the distinction between the allowed and prohibited, the suitable and unsuitable, and no one is able to object or harass him"
This is only true for the true tantric practitioners who has transcended between suffering and pleasure. They know what is suffering, they also know what is pleasure, and they have transcended both pleasure and suffering. In fact, reality has no name and no identity. You say this is pleasure, but this is your opinion. You say this is suffering, but again this is your opinion. You say this is neutral, but again this is also your opinion. Reality has no name and no identity. We can judge reality, but reality is not for us to judge. Reality won't follow your judgement, because reality is free from identity and "pure" from beginning.
Since he has transcended both all social notions, no matter how the whole world condem him or not, he won't be affected, because he knows exactly what is reality. He is no longer swinged by social judgement.
There is always a critical question here: Do you mean if someone who has realized this sunyata can do whatever they please to do?
They won't. Why? Because the nature of sunyata is actually compassion. Anyone who realize sunyata, they will have compassion. They can see exactly how people have suffered due to their mind and those people don't have the ability to see beyond their mind. Enlightened mind see directly if they don't help this ordinary beings, there is no way they can get out from this samsara. People who realize sunyata can no longer differentiate between you and me. For them, there is no such things called I and you. Helping you is no longer equal to "I am helping you".
"I am helping you" is a mindset or ordinary beings who is deluded.
Enlightened mind is naturally helping all beings without I and You.
I am not here nor there.
I am not right nor wrong.
I do not exist neither non-exist.
I am not I nor non-I.
I am not in samsara nor nirvana.
To All Buddhas, I bow down for the teaching of emptiness. Thank You!