Yeah, if some day in the far future I were to move in with my gf and her lovely husband, I would want/need my own bed. Sleeping in their bed with them feels wonderful, but it can just be too crowded for comfort if all I want is to twist and turn a bit and then fall asleep.
Do take Julia's advice and do a tag search for unicorns. It's all too easy for folks with the best of intentions to go about things the wrong way with a third person, and why make the same mistakes as others when you can learn from them instead? There are also some great essays at www.xeromag.com
that may be useful to you, such as "things to consider when dating a couple".
The major players. Me, under-30 bi female. Gia, girlfriend of 3+ years. Clay, new boyfriend/dom. Davis, ex/"it's complicated."
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy, Clay's partner.