Should be, but isn't. Gay couples have been having expensive lawyers write these contracts for twenty years or so and when push comes to shove, they often don't mean shit. That's why we have been pushing for marriage rights.
The other issue is divorce. I know someone who is going through the end of a messy breakup with his long-term boyfriend. They met before marriage or civil partnership were a thing and the process of untangling shared assets, property, mortgages and so on is really complicated.
Yes. It's a shortcut, and I completely understand why people want to take it.
Divorce is pretty easy compared to marriage. Like you said, it's just figuring out who gets what...
No. United State v Windsor (the DOMA case) proved that. Spouses have estate tax exemptions. Windsor sued the IRS for $363,053 in estate taxes that she paid to them because the IRS didn't recognize her and her wife as legally married for tax purposes even thought New York did.
Civil marriage is a form of a "normal contract."
Inheritance should be simple enough with normal contracts, regardless of marriage (same goes for hospital visitation and medical decision-making, which you didn't mention). Immigration is already completely arbitrary, at the whim of the government. The government could change the immigration effects of marriage just as easily as changing who can get married. That brings us to taxes. Well, that's already an extremely obvious violation of the equal protection clause. Why should married people's taxes be any different than single people's taxes?