I'm sorry you don't like my answer. Is it the wording, or the fact that the answer isn't the one you wanted, that is the problem?
No, being a heterosexual domestic partner in the same household does not make you a dependent who is eligible for coverage unless the specific plan says so. There is NO legal requirement that ANY dependants be covered on ANYONE'S health plan. Each group insurance plan spells out who is considered a qualified dependent. The large majority of plans limit coverage to LEGAL spouses and children. Occasionally, a plan will allow for homosexual dependent partners since they do not have the option of getting married. In most cases, the partners will have to provide an affidavit to the effect that they share a household, expenses, and so on. Few if any extend this exception to heterosexual unmarried partners, since they have the option of marrying and becoming a legal spouse. And before you ask, no, this is not illegal discrimination.
I don't know any other way to say it, so if you don't like the way I phrase it I really don't care. Unless your fiance's plan specifically and in writing allows for heterosexual unmarried partners to be covered, which is HIGHLY unlikely, there is no way you are going to be covered on his plan until after you are married and there is no legal way you can force them to cover you. If it doesn't make sense to you to buy a second policy, then don't buy a second policy and go without insurance. If you care about being covered, then buy a second policy regardless of whether it makes sense to you or not. Those are your options.
IAAL, you're a sweetheart. Thanks again; the check is in the mail.