Miley Cyrus and Liam Hemsworth are amused by persistent rumors they're splitting up because they haven't tied the knot ... 'cause going to the altar isn't important and, in fact, it may never happen.

It may sound strange if you're older than 35, but sources directly connected to the couple say the formality of marriage isn't important. What they view as important are ... they love each other, they're happy and enjoy each other's company.

We asked the obvious question ... why then get engaged? We're told their engagement back in October 2016 was a symbol of commitment, but it was enough of a symbol for them. Miley has actually said she doesn't want to get married, despite the engagement. She doesn't think it makes a relationship better.

We're told privately they are as tight as any married couple, with very little drama. As one source put it, "They're both super chill and grounded. They're perfect for each other."

As for the breakup rumors ... they literally laugh them off.