Confused

If gender doesn't exist and is purely a mental construct, doesn't that negate all the feminist arguments? How can one claim discrimination against something that doesn't exist? We can't group men & women together in competitive sports because folks claim men have an unfair physiological advantage. But yet, a 6'2" 250 lb muscle-bound male born person can (and did) select their gender as female and subsequently shatter women's sports records. That's ok, right? The liberals say that only an individual may decide their own gender and nobody has the right to contest it. Someone needs to sort this shit out because it's starting to confuse me.