Gay Marriage

IMHO there should be -

Marriage - for two people who want to get married whether MF, FF or MM.

Civil Partnership - for two people who want to join together in a sexual relationship without having to get married, for wahtever reason, whether MF, FF or MM.

Partnership Arrangement (for want of a bettter expression) - for people who do not want a marriage or a civil partnership but who have a friend or relation who they want to commit to to get the legal and tax benefits (eg two spinster sisters who live together and want the IHT benefits, two friends who trust each other implicitly re: health care in event of debilitating illness and who are not wishing to to be married or in a civil partnership.)

Why can't all these be the same thing?