[MEGoody]

"Why is it imperitive that there be a "legal definition" of marriage"

Insurance

Making health decisions if you are unconscious or otherwise incapable

"Default" beneficiary of estate

Tax returns (joint filing)

That's off the top of my head. I could probably think of others if I wanted to strain my brain.

[mvpel]

Insurance is market-driven - if enough people want a given insurance company to offer benefits to their domestic partners, then the market will produce companies willing to do so.

Health decisions - that can all be handled through powers of attorney and other contractual structures.

Default beneficiary - if you don't have a will, you should, even if you're married.

Tax returns - subjecting ourselves to the raft of BS social engineering projects that our legislators have foisted off on us through Form 1040 and its adjuncts. If we had a fair tax system, it wouldn't matter whether or not you're married with respect to your taxes.