Couple as Guru
An important tenet when it comes to Tantric sexuality (specifically under Neo-Tantra beliefs) is the idea that romantic partners serve as each other's guru. Therefore, the partners need to have total and complete trust and faith in each other. It is critical in a Tantric relationship to have unwavering faith in your partner, to believe that he or she has your well-being and spiritual happiness utmost in his or her mind.
If it is established (or assumed) in the relationship that both partners will remain faithful and one partner violates that rule, this can result in a major breach of trust. The partner who has been cheated on may feel betrayed, and feel that his or her partner has violated the sacred trust of the relationship. In essence, the actual infidelity is secondary to the more important issue of trust and honesty.