I think it's lovely that so many people wouldn't have a problem with it because they trust their partner.
But some of us are more cynical because we've had some experience of how trusting someone unconditionally in the past has backfired, or we have good friends who have experienced it.
How many women post on here because their lives have been turned upside down because the partner they trusted, the good guy, the family man they've known for decades, has embarked on an affair?
How many times do women post that their partner doesn't love them any more but there definitely isn't an ow, and she believes this because he'd never ever do that? And then she finds out that there was an ow all along.
It is actually pretty offensive to insinuate that those of us who understand how affairs start, even in happy marriages, are simply over-protective or have picked the wrong men.
We are not talking about a one-off here, they are working away together regularly, sitting in a bedroom together regularly.
This is how my stbxh's affair started. Someone he had worked with for years without attraction or incident. Suddenly they're on a project together and working away often. Over a year their relationship, through shared experience and conversation, through gradually discussing more private things, through spending more downtime with her than with me during the working week, developed into something inappropriate.
I think it would be respectful of op's partner to remove any real or perceived temptation by avoiding being alone in a bedroom with his work colleague, it's not even difficult to do.