Something New
Random thoughts here....
1) It is something new and exciting and unknown. Was it really Robert she was in love with or the whole situation - talking, he makes her laugh, he helps cook etc. I'm sure it's been talked about before but as I rewatch I want to ponder some more. She doesn't know any of Roberts bad habits yet, nothing. All the things that make real relationships "real." It seems to be more of the idea than the actual person....
2) it's always "the boring marriage." What marriage is all fun and excitement? Life isn't that way how can a marriage be? The affair is the classic fantasy of "something new and fun and better than what you have" Maybe when the family came home from the fair she could have tried to spice it up?
Wouldn't life have been "hard" with Robert too? Constant traveling can be just as bad as being stuck in one place. No comforts of home, no roots, exhaustion etc. I imagine Robert would be off in a jungle somewhere and Francesca would be back at the hotels figuring out dinner, etc. it wouldn't be all glamour. Kinda like how so many celebrities get divorced with all the money, private planes, jet setting to tropical places. Anything can eventually take its toll.