Right, so long as its not my offspring that you are willing to sacrifice. And everybody has this same viewpoint for his own value of "my".
You chose to use the word "everyone" ; that is a word with only one meaning.
You are wrong. It is absolutely and incontrovertibly untrue that "everyone has this same viewpoint". I do not hold this opinion. The two-decade old receipt for my vasectomy (before having any children ; it was a bureaucratic struggle) supports my assertion that I hold a different opinion to you on this matter. I can also think of at least ten others of my friends who do not have children and who assert that they do not want to have children ; several of them are at or beyond the technical limits of child-starting age and remain child-free. This also supports my correction of your claim that "everyone" thinks like you. "Every-", "all" etc are words and prefixes that you should think several times before ever using.
Incidentally, I object to paying taxes to subsidise your children, their education and your spending on food and clothing for them. I'd rather spend the money on development of elder-care robots and extending lifespans. Robots are considerably less resource wasteful than people. Since I do get out and vote, this might be an incentive for you to do likewise.
But "Backup Earth" is not the only reason to go out colonizing.
"Backup Earth" never has been a credible reason for going out to colonise, in any sense of possibly providing a place where Earth-born humans can go to in any demographically significant numbers (for Earth ; far smaller numbers would be significant for the putative colony). The number of people who will ever die on a planet that they were not born on is always (caveat follows!) going to be far smaller than the number who die on the planet of their birth, for the same reason that today most people die in the country of their birth : transport is expensive. Colonies rarely receive more than 1% / year of their population by immigration - most of their growth is by local breeding of second and higher generation natives. Meanwhile the colony's internal growth can exceed 3% / year. Those numbers add up.
(Caveat : assuming that the currently-understood laws of physics hold, in particular the equivalence of inertial and gravitational mass, and the speed of light being an upper limit on speed. Actually, it doesn't matter that c is "legal max" ; even getting to c/10 qualifies as "expensive".)
If we (our generations) conspire with your children to fuck up the planet for their children, then it is your children's grandchildren who will suffer on Earth in consequence. The odds of your descendants including anyone who gets off planet (e.g., to the asteroids) are low (the corresponding probability for me is zero, of course).