You are correct about the pretention. Isn't it also a little pretentious to "believe" that we aren't alone? It can go both ways.
Actually, no, it's not, based on probablities as well as ego. Saying "we are unique" is obviously alot more pretentious than saying "we are not unique" - the first sentence implies special status, the other does not. Additionally, believing in something is that is VERY VERY VERY probable, mathmatically, versus believing something very very VERY improbable is pretty much the opposite of pretention - it's common sense.
If one believes in life evolving through chance chemical interactions, then it is short-sighted NOT to consider that life could have evolved elsewhere. Given the billions of galaxies, each containing billions of stars, then the sheer magnitude of star systems suggests that, if we are the only life forms in the universe, we are extremely lucky. We are the Powerball winners of the life lottery.
I can't say for sure that there is life elsewhere, nor would I myopically state that we MUST be the only ones. As the saying goes, absence of evidence does not mean evidence of absence. That applies to the scientific view as well as the religious one.
I don't "believe" one way or the other. All I do is search. If we are alone that is momentous, if we are not, that too is momentous. SETI is win win science.