There are usually several aspects to the cloth issue that I have identified during my past 6 years of work:
First, there is no "must be done". Tantric massage does not have a set of rules, so each Master works in his own way.
The absence of clothes on the Master is not a necessary condition for immersion in a meditative state and unity with oneself. Nudity of the Master by no means helps the client in this aspect.
Lack of clothing, in my opinion, is acceptable for a tantric couple, where the participants are equal. But invalid for the Master running and supporting the clients' process.
When the Master is dressed, this creates a space of Master neutrality in which the clients' healing can occur, as his mind is no longer distracted by the naked body.
A dressed Master is ethical. I work at the level of the heart, of the soul - in the process of practice, psychotherapeutic transference can occur, and working with it without clothes is unethical and can lead to unpleasant consequences.
When the Master is undressed, this creates unnecessary images in the client's head and one loses focus on oneself. Instead the focus switches to the body of the Master and objectification occurs.
After all, I don't like being a sexual object. You come here for your healing inside a beautiful and sacred Ritual, not sexual stimulation, right? If you are looking for sexual stimulation - please, continue your search - I don't do that.