I think you're a little confused about the meaning fo the word 'monogamy'.

When you say:

"because a person might be monogamous but that does not mean that he has only one sexual partner" then that person would be defined as polygamous, or possibly promiscuous (the two words don't mean the same thing).

My concise Oxford dictionary defines monogamy as:

"the practice of being married to or having a sexual relationship with only one person at a time."

So if you are married to one person, that is the only person you are having sexual relationships.

And if you are not married, you are only have sexual relationships with a singlec de facto partner at any one time.

I hope that makes sense.