No man (except for a very little few, like myself) would admit that there is some inherit instinct by the part of males to try to build up their own sort of harem when circumstances allow.
Monogamy or the idea of one-on-one relationships is something very culturally constructed over the course of society and social experience. Not a natural behavior expected in social animals such as humans. That is not to say that all men are equal in this, but never forget that most men do not dispose of the means to build their harem. Being them material or social acceptance, etc.
The truth is that from all that you said, it mostly points out to the fact this guy was ‘keeping’ you as sort of insurance… in case things went south over the other girl, and if things continued to work out with both of you, there is nothing that naturally breaks that behavior, of course the moment you one in the pack questions this, then it becomes a estranged situation for which most men coward and run keeping with the prospect with lowest odds.
It all comes down to respect for the other part, without that, behaviors will not flow naturally. If a relation is to be one-on-one it takes a lot of respect to keep it from being the usual traditional promise meant to be broken.