We can quibble about "basic human right", but since it affects deathbed decisions, inheritance, and taxation "marriage" affects a lot of rights.
The government is involved because of the aforementioned issues, since we have jointly decided to give our government some say-so over those issues. We can argue if this is wise, though I'm not sure how else we could resolve those issues.
It is limited to two people partly because western civilization has had only two-person marriage for over a thousand years, party because 3+ person marriages raise some new issues that would have to be worked out, and partly because most western polygamous groups have used it to award underage girls against their will to old, powerful men. I expect that 3+ person marriages will come in a few decades.
I'm not sure that boycotting Firefox is the correct action, but consumer boycotts seem pretty common by all sides of this issue.