I think that a court would probably be open to the suggestion that adultery would count as "unreasonable behaviour" - though the question might arise as to whether "adultery" was a legally correct word for it, being a word that is traditionally associated with the term "marriage"

I don't think I've ever heard it used in that context.

Would I be correct in assuming that people who favour the word "marriage" being extended to include single sex relationships would see "adultery" as needing the same extension?