Heterosexuals cannot marry just anybody they love. They cannot marry someone under age. They cannot marry someone closely related to them. They cannot marry someone already married. They cannot marry someone if they are already married.
If you want to do away with all these laws and make it open season, that is at least consistent. But it makes a mockery of marriage and no society has survived long without a generally agreed on definition of married.
Homosexual marriage will certainly be bound by the same prohibitions. And heterosexuals may marry someone under 18 in most states with the consent of the parent or a judge.
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Why? Why should they be bound by any restriction? Why shouldn't someone marry their sister if they want? (This is an actual case working its way through the courts). Why shouldn't someone marry 4 people if they want? In Muslim countries this happens all the time.
Once you do away with restrictions there is no reason to keep any of it.