I don't see either of you as "in the wrong" on this one - some people enjoy longer-term, some shorter-term. Both are perfectly viable possibilities for relationships.
Is there an expectation for the two of you to "share" your relationships with each other in some way (other than the knowing about it and maybe being friends with partners)? If there is, then you two definitely have some talking to do to hash out how this short work with your differing priorities.
If not, then why is it important to him that you lead your love life according to the others standards and criteria? Can't he love the way he wants to and you the way you want to? What would be the fundamental concern with this.
I think that this is a dialogue that the two of you most definitely want to have - to understand what the underlying concerns are, and to talk them through and deal with them.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb