Kind of a few holes:
Why would the Alien not perform exploratory actions on Dallas and Brett?
Brett's egg seemed to be developing just fine. If it were a dud, why would it not check Dallas to not repeat the mistake?
How can it even detect human ovaries? Specifically, ones that are developed enough to ovulate -- human females are born with all the eggs they'll ever have in their life, they don't appear at puberty.
Facehuggers don't care about age or sex. Newt was closest, Ripley the next closest. The dog was male.
Good questions! Firstly, it's entirely possible that it *did* check Dallas and Brett once it had them - certain species of parasitoid wasps will repeatedly 'stab' potential hosts *after* they are cocooned to check if they are suitable.
As for how a xenomorph would differentiate male from female hosts - it may use olfactory signals (chemical cues) many species rely on contact cues in reproduction - it may have had to physically touch with its tail to assess.
The dog, in this case wouldn't have mattered, as a queen embryo was already laid in a suitible host for its developmental requirements - the runner was a simple drone, any host was suitable. :-)
Theres also peer reviewed papers that conclude that certain parasitoid species have a sex bias when it comes to hosts - this was one of the first ones to pop up on google where female hosts are actively pursued:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3823929/
Why does the Alien attempt to head munch Ripley if she is so important? Same reason the runner frantically attempts to attack Ripley whilst she is carrying its future after molten lead is poured on it - self preservation.
As I say, it's just a personal theory, though Prometheus and Covenant have since heavily played into it, especially David's drawings 😊