Not specific to any situation in this thread
I want my son (age seven, has a diagnosis of autistic spectrum disorder) to be accepting of all types of relationships. I think a way to do this is to be very careful with how you explain romantic relationships from a very young age. For example, I have always said to my son that any adults can fall in love, marry and start a family together if they wish to. I told him this at 4 when he asked me who was "the girlfriend" out of a close family friend and his boyfriend. He could tell they were in a romantic relationship despite not seeing any PDA's and said that he thought that E, who is actually the "bottom", was the girlfriend. That explanation satisfied him and now he doesn't view homosexual relationships as alternative, especially as he has been to same sex weddings and knows a good few homosexual couples, he sees them as normal as heterosexual couples. Homosexual celebrity couples such as Elton John and David Furnish who publicly adopt or use surrogacy or artificial insemination to start a family offer opportunities to show how a homosexual relationship does not mean that one has to sacrifice their wish to be a parent and that the things that make a family are love, loyalty and companionship rather than the gender, beliefs or lifestyles of the people involved.
Recently he referred to something on tv which was actually a form of ethical non monogamy in the sense that both women were aware their partner had another partner. He described this as cheating and I used that opportunity to explain that it could only be cheating if any of the people involved were unaware of the existence of the other relationships their partner was involved in. That leaves the opportunity to later explain that someone can still cheat in a non monogamous relationship by breaking agreements.
My son now knows that adults can have as many relationships as he wants, with people of whatever gender he wants, as long as he is honest and forthcoming with the people he has those relationships with. This is the norm I have created for him. This is what other ideologies will have to compete with.