This isn't really relevant to your question at all, but you reminded me of a (male) friend who is a gynecologist and married to a woman. I expected that the professional context would nullify any potential arousal towards his patients, but what I was curious about was whether this might bleed over into his personal life — i.e. did he still find his partner's vulva arousing, or does it put him into doctor-headspace. Apparently his profession causes no problems whatsoever in his sex life, because the compartmentalisation is so strong.
He said that it feels almost like conceptual homonyms. For example, in the sentence "up past the river bank is the bank where I deposited my money", the word "bank" appears twice but means two very different things. Similarly, a vulva is a vulva no matter the context, but the meaning of it differs so much depending on the context that his brain literally doesn't parse them as being the same.
Like I say, it's not related to your question, but I thought you might find it cool nonetheless. I would expect that firefighters would show a similar ability to compartmentalise, but perhaps the high-stress context of smelling human flesh may cause it to work differently.
Sometimes the driving force is a bi-curious woman. What usually happens is that the boyfriend agrees to it because he sees a MDF threesome as being hot, and sapphic love as being less real or serious. Then he freaks out during/after the hookup because of insecurity he feels when seeing his girlfriend enthusiastically making out with a woman. I've learned the unpleasant way that it's no fun to be unicorn hunted.
The worst part is when they try to hide what they're doing. I once only found out a woman had a boyfriend and that they were looking for a MFF threesome on the third date. Trying to hide their intentions is gross because it shows they have some awareness of how people don't like being instrumentalised in this way.