He's just more into science/reality and thus doesn't believe in all the multiple gender insanity nor weird anti-science nonsense that pretends men can transform into women (or vice-cersa). Being pro-science and pro-biology can only be a good thing.
1. No, Trump isn't into "science/reality". If that were the case then the whole entire
COVID-19 nonsense he was spewing around would not have happened. If anything, he is not above spreading misinformation to the public even when he knows better.
In an interview with Woodward on 7 February 2020, Trump said he knew that COVID-19 was more lethal than the flu and that it spread through the air. "This is deadly stuff," he said. But on 9 March, he tweeted that the "common flu" was worse than COVID-19, while economic advisor Larry Kudlow and presidential counselor Kellyanne Conway assured the public that the virus was contained. On 19 March, Trump told Woodward that he did not want to level with the American people about the danger of the virus. "I wanted to always play it down," he said, "I still like playing it down." Playing it down meant lying about the fact that he knew the country was in grave danger.
2. Gender is a social construct. This is a view that many in academia hold and denying this is going against reality.
Gender is a social and structural variable that encompasses multiple domains, each of which influences health: gender identity and expression, gender roles and norms, gendered power relations, and gender equality and equity.
While many cultures view gender as a binary, some cultures, both past and present, view it differently.
Take the Hijra or the Sekrata, for example. Even if we were to make the whole "gender=sex" argument, the whole gender binary thing would still fall apart since the many scientists view
sex as bimodal rather than as binary. In reality, the genetics behind sex are actually extremely complex leading to many pushing against the idea of viewing sex in a dichotomous manner due to it being too simplistic. Even a professor at my university for my first year biology course talked about it being better to view sex as bimodal/as a spectrum rather than to view it as a binary. Going by this logic, viewing gender as binary would still not make any sense.
3. Being pro-science and pro-biology means actually understanding that most things in biology are incredibly complex and that not everything is 100% determined by biology and that we can still be influenced by different aspects of our environment and culture (nature and nurture). Humans like to categorize things because it makes shit easier for us to understand but the way how we may categorize things can be arbitrary and can differ from culture to culture. Viewing gender as a binary is incredibly restrictive and is largely based in culture rather than in science. Even if we were to view gender as biological it still can't really be viewed as a binary since many scientists don't even view sex as a binary. At bare minimum, there would still end up being a huge debate over whether or not it is a spectrum or not.
I don't get why people expressing their gender in ways that don't fit the binary is a bad thing. I think that being able to express yourself in a manner that makes you comfortable is perfectly fine so long as you aren't hurting anybody. Humans are weird, nature is weird. We are complicated being so it's only natural that some may find the gender binary to be too restricted. If having more than two genders allows others to feel more free and less dysphoric and achieve gender euphoria then I don't see what the big deal is. I think that diversity in gender expression is a wonderful thing.