How hard would it be to take another co-worker to the lunch too? Then they would not be alone.
The fishing trip would be different too, There is a group of guys, they wouldn't be alone. In my example of ds and dil, they don't go on fishing trips. They go out to dinner and for drinks with a group of friends. She goes out with a group of females. Unless one brought her husband, they wouldn't bring a guy. But if one did, ds would never tell her she can't go. She would choose on her on.
Play group, not alone.
You are making it into something its not.
Well, I was responding to this part of your comment:
" They each have time "out with the boys" or "the girls",
they just don't go in a mixed group unless they both go. " which made it sound like just other people being there was not enough; it had to be ALL single sex or else both memebers of the couple had to be there.
As for "how hard would it be to take another co worker to lunch" as others have explained that can be hard. Here are two real life examples:
My DH travels extensively to many factiries. He is the lead/director for his department and able to handle all types of issues at a high level. They have 12 "experts" in just one of the many issue types. These are all younger/newer people who have other jobs as well but have learned enough about this one topic to go in and train some or lead some projects. The first year or so that they have that designation they will go along and shadow DH when he handles their topic, and hopefully contribute more and more of the leadership over the week so that after a couple such weeks they can take on many of the smaller/easier projects and that goes off his plate. So DH, and that person usually arrive at a plant on Monday late afternoon after a day of travel (which often involves 3-4 hours driving to some middle of nowhere place, in a shared rental car). They either stop for dinner in route, or go to dinner shortly after checking in. At that point they won't have been in the plant (that trip) to even ask someone to join them and generally the locals will not be jumping at the chance to miss a night at home to entertain them--besides which they need to go over the game plan for the next day, and there will be no one locally who would make sense to be a part of that. A couple of times during the week there will probably be big dinners involving lots of people from the plant but there is likely to be at least one other night in which no local really wants to play host, and DH can offer lots of good feedback and help to the person in that one on one dinner and mentor them and help them get better at their job.
It's a male dominated field, but two out of the twelve in that group are women---both he and I have been happy that some women are breaking that glass ceiling a bit.
Also, DH is in charge of Europe and Asia, his US counterpart is in charge of North and South America and is a woman. 2-3 times per year they get together to work on global strategy and make sure they are working at the same level, etc. They fly to one of the plants (or, better, an area where they can get to a couple in the week) and make sure that they both are seeing the same issues in audits and scoring the same, etc but then also have working dinners to discuss the strategies, training ideas, etc. It wouldn't even be appropriate for the local workers or plant managers to be in on these meetings as that could be seen as giving the visited plant an advantage over othes, etc.
So there are two real life examples of how it can be appropriate and necessary to have one on one bussiness dinners, for just my husband and he is far from unique.
My comment was meant to convey that if someone has a relationship rule in which they choose not to be in such situations I should hope that person would limit themself and not accept promotions, or quit and work in a different field, etc rather than end up in a position like my husband and then, say, suggest only men for promotion into positions which the person is required to meet with, or bow out of all those dinners with the female mentorees leaving them unable to learn as much as hteir male coutnerparts and then looking less qualified to upper management. Basically, if you are going to place those rules on YOUR relationship, I hope you (general you) are very cognizant of how it could limit others and actively work to be sure you are not allowing it do so---but rather making sure if anyone is limited by your rules it is yourself---especially when it comes to career advancement.