From someone who knows this subject well in many areas,i will give you some advice.Ill do this in sections so its easier for me to
explain.Now many dont understand that once more then 1 woman or man is entered into a relationship,it is not seen as open but poly,because there is either 1) more then one commitment or 2) more then one person sharing the same interest.from what you explained in your post,this is what *open* means,it means that both agree to openly have multi *sex* partners or whatever they need,this does not mean other
relationships.So it sounds like to me he is either pulling your leg or there is some mis-communication going on.That aside ill explain from all those aspects.
1) Poly-Commitment with more then one person at once.NOT something to get into if your one seeking a commitment with one person,a jealous type or one who doesnt understand its point.A poly commitment isnt really about sex or anything of the such but more of
emotions.It is seen as a family,of course sex does take place if they are seeking children but i dont advice it unless you know yourself well enough to be ok with sharing.
2) Open-same with sharing but your commited to one person with
benefits.you both wish to enjoy the pleasures of life while coming home to the one who has their heart.tho this is mainly sexual,it depends on the couple and what they feel they need from someone else other then their partner.again,i would not advice this unless you know yourself well enough.
3) Committed relationship-basicly what it says.two people only,sharing only each other and saying fu** the world and everyone in it.knowing they only need each other and nothing else.This is the best choice for someone seeking love and commitment and nothing else.
In a nut shell,all of them can break a persons heart but the rate is higher when there is more then one person involved.unless you know all party's THAT well,you will always feel ignored,2nd best,love will always get involved and things will always
fail.so all i can say is,know the situation,know yourself and them then act on what you want.