I guess I'll drop my own insights here as well.

I am personally polyamorous, and have been for as long as I can remember. I have to agree with everyone that says yes, you should absolutely tell the people in your life something that important. There is a fine line between keeping certain things private and lying to those around you. And I have to agree with the assessment of your Dom.

Supposedly you meant so much to him that he wasn't willing to share you, he was married to another women - something that usually implies a hi level of love as well - yet you two had no idea about his 'real' life. I can't see how this would work out in your favor. I don't want to think about how things would have ended changing after a few years.

As for asking about all Doms, there is no absolute. We are 6.5 billion individuals, and every one of is going to have different rules about life.