One way to understand that is it simply isn't true. Mars and Venus don't necessarily have to represent which gender we're attracted to or how. Not all astrologers use them that way. In fact, those particular uses of Mars and Venus are increasingly falling out of favor. They don't work without a heteronormative assumption, and fewer astrologers these days are assuming heteronormativity.
Another way to understand that is to see Mars as your ideal man if you're attracted to men, and Venus as your ideal woman if you're attracted to women. If you're attracted to both genders, then both of those expressions of Mars and Venus are "on" for you. If you're attracted to just one, then one of those expressions is turned off. So, if you're attracted exclusively to men, Venus doesn't indicate your idea woman, although all the other meanings of Venus in your natal chart apply. If you're attracted exclusively to women, Mars doesn't indicate your ideal man, although all the other meanings of Mars apply.
That method doesn't rely on the gender of the querent, but on the gender(s) they're attracted to. But, it would still be hard or impossible to use if the querent and/or people they're attracted to are genderqueer/non-binary.
There's a whole field of queer astrology, and one of its takeaways is a more universal, non-gendered use of the planets, in a way that applies to everyone no matter what gender or sexual orientation they are.