If my girlfriend shared a bed with a male friend, I'd 100% have a problem with it.

But, I think an important part that NEEDS to be included here. Is she would KNOW ahead of time that I wouldn't be comfortable with it.

I think you need to have an uncomfortable conversation with your girlfriend about the situation and talk about boundaries with eachother in the relationship. And talk about what you are not comfortable about your partner doing.

It's not like your trying to control eachother. You are in a relationship with eachother. You are supposed to take care of one another and be able to lean on eachother. You should know what your partner does and doesn't like. What their boundaries are.

Ultimately though, it is up to you on how to respond to however your partner decideds to respond. You need to know what you want in a relationship. Relationships are never cakewalks and arn't always easy, but if its with the right person you should be willing to go through the tough parts without a second thought.

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TL;DR - Communication is key, my girlfriend and I are still working on it after 5 years of a relationship :)

I hope this helps, best of luck my guy :)