what makes you think not wanting to spay is a "red flag" to them? do you think "he totally fucks her" is on their mind instantly when you tell them you're not interested in needlessly mutilating her?
and what do you mean by "vets know when something has happened"? as in, they "know" when she had sex? unless she's oozing your fluids upon arrival, they "know" jack shit.
it simply boils down to "my animal, my decision" in "normal" people's language
also, some ppl really seem to need to realize how low (if at all) on "normal" person's mind sex with animals really is... there's a reason it's always such a big and shocking news (and laws get pushed immediately) when someone gets caught doing it. it's just so far out of this world for an average person. you can also just look at any average kinkster here, arriving with "yesterday i had no idea it's possible, now i'm hooked forevah"... just be yourself and don't look for "red flags" where there aren't any.