Sandro getting sandwiched between his best friends. Nei'tale and Leonenn were a couple who both expressed interest towards the red head. Sandro expressed interest since they were already friends and felt not just comfortable, but Star would likely not notice anything unusual in their pre-existing friendship.
While in the cult, it wasn't unusual for members to sleep with one another. Each other was all they knew as outcasts of society. Intimacy was encouraged, believing that passion brought out the magical potential of others, even if romantic feelings were not involved.