Are threesomes with another woman a bad idea in a relationship?

My current girlfriend if bisexual and while talking about kinks/etc and threesomes came up, and while im not interested in another guy coming in she said that another girl would be fun. Whats yalls opinion of that? Is that a bad door to open or is it fine since we both want it?
Are threesomes with another woman a bad idea in a relationship?
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