I really don't see the problem tbh. This winter I went up north to go on anice climbing trip, I went with a girl my age, a married guy and a married woman, and they werent married to each other. Maybe this type of trip is different, as people usually don't have a high sex drive after walking through the woods and climbing ice all day, but it really is all about trust. If the person loved their SO, they'd go on the trip or do whatever, and go through with it without a hitch. If they themselves wanted something 'more' and ended up doing something with the person they went on the trip with, who's to say they wouldn't have done it anyway, regardless of going away together on a trip?

But I really don't see how it is inappropriate? If the person I was with had an issue with me going somewhere with a friend of the opposite sex, especially one who I knew before the relationship had started, the person I'd be seeing would probably be getting the boot.