You could also stop thinking you are obtaining someone to enter your family and what you have and start thinking that you will be entering their lives and their family... they will also be obtaining you.
Have you looked at any other threads on this yet? Done a tag search for "secondary" or "third" or and others that you can think of? There are a good number of people who were considered secondaries that were left out in the cold in terms of love taken away, neglected emotionally and abandoned because of the mentality that you are portraying.
I know you have good intentions, or think you do, but you are not obtaining a puppy dog. You are considering joining lives with another who has equal rights to you, is just as intelligent and has a life just like you. I suggest you think in terms of considering what you can give to them when the going get tough (and it will), what considerations and commitments you will have to someone you share a life with, how will you honour the path that they are on and will be on with this.... how you can think of this in terms of stepping outside the couple-centric box you are coming from.
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