That does not seem so far fetched to me. So many of the purported alien sightings are beings with bilateral symmetry, two eyes, two arms, humanoid face and so on. The only way I could see that happening is visitors from a distant future.
Or, more likely, the alien "creators" have created them more or less in our image.
Another possibility is that the Galactic Federation has a rule that when they need to visit a planet that is not yet aware of aliens the crew must entirely consist of beings that have the same general form as the people of that planet.
That makes it harder for someone who sees them to convince others that it was aliens and not just a trick of bad lighting or someone with deformities or injuries that give them an unusual appearance.
It also makes it less likely for them to be mistaken for some other species on the planet. Suppose the Federation sent an expedition to ancient Earth that included crew from a species that looked a lot like our cats, and accidentally let Earth people get too good a look at them and their technology.
Those Earth people might think those are Earth cats, and conclude that Earth cats are a lot more powerful and advanced then they thought, and that they had better stop treating cats like animals lest the cats decided to wipe them out and start treating them as superior beings. Next thing you know that entire civilization is treating cats as magical being of great value or even worshiping them as gods.
It's quite possible that the humanoid lifeform is optimal to have a technological species that can travel between stars. An aquatic species would have huge difficulties just building technology and civilization, because of the habitat. A species without arms and thumbs would have a hard time manipulating its environment (just look at all the 4-legged animals now). A species with more than 4 limbs would likely either be too small (insects) to accomplish much, or would need too much energy (and probably evolve to lose the extra limbs over time).
There's good reasons to think that alien species might not look all that different from animals on this planet, simply due to physics. Animals here didn't simply spring to life in their current form; they evolved from single-celled organisms to best suit their environments.
The word 'parochialism' positively vaults to mind. Not to mention your flagrantly unjustifiable opening assumption, given the number of bipedal technological species known to us to travel among the stars is currently zero.
Speaking of Golden Age twists! And not fully thought-through ones, at that. It requires two assumptions: first that there exists a yahweh-style creator deity, and second that Genesis 1:26 is accurate to fact. Even taking both as axiomatic, this approach still further assumes that this likeness, namely the one in which we as humans are made, must also be the only likeness in which a mortal could be made after its creator.
Given the assumptions of faith under which we here labor, it may also be wise to heed 1 Cor. 2:11, in which the convert Roman makes one of his few worthy statements in warning men against imagining they can know the mind of God. In that light, the proposition lacks soundness even under its own axioms.
You're kidding right? It's all SciFi. In case you're confused, the Fi is short for Fiction. Stuff that's not real. So of course we're making assumptions on the entire thing. Including The Book as the greatest selling book of fiction of all time.
You're also now assuming that we Earthlings are the original source. Some scifi tropes state we're more Martian fleeing their dying planet or with things like panspermia. I like the SciFi where everyone is searching for the nearly mythical planet that turns out to be Earth. Ice Pirates is a goofy one.
You may labor under a misapprehension here; if I met Yahweh on the road, I would do my level best to kill it. But I was raised with that book, and still remember enough to play with the toys in it when I want to; if we're talking 1950s sf twists like "the aliens were fellow children of God all along!" then those are the toys with which we're playing.
That aside, of course we're making assumptions. But if we don't choose to either be bound by the assumptions we've already made or re-evaluate them, then we're playing with dolls rather than worldbuilding. Your pastimes are of course your own business, but it's been a long time indeed since I graduated from the former to the latter.
(Not that I mind space opera, when it focuses on the character-driven stories it's best suited to tell - trying to figure out how a TARDIS works misses the point entirely, while "The Doctor's Wife" is beautiful. But you mentioned science fiction, and my current standard there is set by Children of Time and Blindsight.)