You’re absolutely right. Women are more than willing to share the attentions of an alpha male, rather than have the exclusive devotion of a lesser beta male. Harems already exist for both single and married alpha men; you just don’t publically hear about them.
And you’re absolutely right! Alpha men DO have harems except that it’s socially acceptable for them to say that they have a wife and a couple mistresses on the side. For corporate CEO types it’s almost a social requirement that they have a mistress.