The relationships described in the Trilogy, though predominantly traditionally heterosexual in nature, does include bisexuality and gay couples in non-traditional arrangements. Most people are aware of polygamy, polyandry, and open marriages, but fewer are aware of the trend toward polyamorous arrangements.
Polyamory connotes the stable relationship which is monogamous between three or more adults.
noting or relating to polyamory, the practice or condition of participating simultaneously in more than one serious romantic or sexual relationships with the knowledge and consent of all partners.
It is distinguished from an open marriage because the arrangement is not “open” or transitory. In the example of a three-person polyamorous arrangement, there might be two women and one man or two men and one woman. But the relationship includes only these partners and no other. The triad may live together, or other arrangements made. Due to the novelty of the arrangement, it is difficult to define with precision.
The Trilogy has such an arrangement which begins in Cryptid Probe and continues through the end of Cryptid Resolution. It should not be understood as being included in the Trilogy not purely for purposes of titillation, but deeply engrained in the core story line. What will be clear in reading Probe and Resolution are the reasons behind such a relationship, like puzzle parts connected to see the whole picture.
However, I would note that I have known of the existence of such relationships within my circle of friends, relationships that have endured for friends of mine for many years. The departure from traditional monogamous relationships are often subject to ridicule and social opprobrium; however, I have never known any of my friends to discuss their relationship openly, nor to advocate their lifestyle to anyone.
The inclusion of gay, bisexual, and polyamorous relationships is meant to reflect changing attitudes toward evolving human relationships, sexuality, and non-monogamous arrangements. This is reality, and rejecting any form of adult, consensual arrangement is the business of nobody but the participants. For those who have expressed these arrangements as deviant, I might check that attitude.
If this aspect of plot is somehow shocking, then you have not lived in the world, at least my world. I have been fortunate that friends have been willing to share descriptions of their arrangements in confidence; to be clear, I have never been invited to participate in any of these existing relationships.
Society is changing and evolving. Ten years ago, I would have thought gay marriage impossible in my lifetime.