You know, I had typed out a long list of suspicious observations about this couple which have lead to my conclusions about the status of B's relationships
. Purple, they go way beyond what you have described. I also work in a male dominated industry and have male dominated hobbies, so I get that there isn't always gender segregation, and I don't immediately leap to the conclusion of affair just because mixed genders get together. I have also been doubtful of the impartiality of other posters, so I get that too.
I already feel a bit uncomfortable about the amount of data that I have shared at this point, so I will summarize on the topic of the "affair" (if there is one). There is a surplus of suspicious observations of the couple. I'm declining to list them all, but when I started, I got up to 12 with more to come...I will concede that they have never kissed or touched in front of me (but neither has my sister and her bf - PDA isn't done in our group). I will also say that my sister's BF is B's close friend. When BF questioned him about what was going on with C, B's response was "I don't talk about that" If there was nothing going on, why not deny?
Anyway, putting the affair issue completely aside. I think it is at minimum fair to say that B and C are not an established or announced couple (married, engaged, living together). They are not
a social unit. They are not afforded the privilege of assuming that each is included/has the right to be included in invitations to the other to social events. B responding to invitations where only he is included with "C and I" is not in any way sanctioned.
We would probably have tripped happily over and moved on from that faux pas if we liked C better.
Now, I'm wondering the following. Let's say that A does not like C. And he does not appreciate that B invited C, and put C on the "To:" line of the email instead of just asking A privately if he could add C to the guest list. What should A do? Should he email B privately and say "I did not invite C. Please, rescind the invitation. She is not welcome at my event"? And let's say that A suspects that others don't like C and that's why they might RSVP "no", how would A communicate that to everyone else? Should he email everyone privately and say "In the future, if you reply to all, please, ensure that C's name is not on the 'To:' line because she is not invited"? Or should he say nothing publicly and just let most of the people RSVP no and then he just has a tiny little gathering? Or should he cancel it outright, and then issue a new invitation (maybe for a different date to present the illusion that a date conflict is why he canceled the first gathering) to everyone except C? And except B?
Actually, the second scenario happened just a couple of weeks ago. B responded affirmatively as "C and I" and added her to the To: line quite early. And...every subsequent RSVP was no. I am a little more interested in how you all think A should respond.