> Are we not actively trying to terraform Mars?

No

> Why is "life contamination" seen as a problem for our solar system rather than a goal?

Because you'll not be sure if eventual life that you found there comes from Earth equipment or was there in the first place

So, why is it a goal to send humans there? Humans breathe out bacteria constantly and will guarantee contamination of Mars.

The days of colonialism, when Europeans showed up on foreign shores, killed the natives, and claimed the land as their own, are over.

  are over

You mean like in Iraq, Afghanistan, Syria, and Israel?

Those are proxy wars and/or puppet governments, which is a different thing to colonisation. There is somewhat more regard for the rights of the native inhabitants than in the past.

If we ever find alien life, the Outer Space Treaty prohibits colonisation in the sense we're using here. Of course this is a very easy treaty to sign up to at this point in our development of space travel. If space travel ever becomes cheap enough to enable exploitation of resource on distant planets I imagine those laws will see some amendments.

So, why is it a goal to send humans there? Humans breathe out bacteria constantly and will guarantee contamination of Mars.

*barfs