The problem with that is, what you can or can’t “picture” has no impact on what might or might not have happened, had a male had that part. And, we’ll never know, because that didn’t come to be.
So, instead of being an argument FOR the obvious, purposeful sexualization of the movie that was actually filmed, this is really just you stating your supposition of what the intent of the makers of the film was, based only on your own idea of what they were aiming for.
Regardless, the scene, as written, had Lt. Ripley change out of a jumpsuit and into an environment suit, preparing to vent all air out of the escape pod and space the xenomorph. I don’t know what kind of underwear a male Ripley would have worn, but I’m confident that you’re correct…it probably wouldn’t have been women’s panties. Although, you never know. :)