It is well known that cisgender gay men are more likely to have older brothers versus cisgender straight men. Of course this isn’t true for everyone but it is on average.
Researchers call this phenomenon the fraternal birth order effect or FBOE, and it’s been relatively well-established across 30 different studies over the past three decades. (There is no equivalent effect among lesbian versus straight women; to the best of our knowledge, this is a strictly cis male thing.)
There have been two recent studies that suggest that FBOE is particularly strong among gay cisgender men who identify as bottoms versus tops or versatiles.
Since studies have also shown that bottoms are on average more feminine in many personality, behavioral, and cognitive traits than either versatiles or tops, these new findings suggest that the FBOE might affect sexual orientation specifically through increasing gender-nonconformity.