That weird caveat about right handedness actually makes me think this is a spurious correlation.
That sounds so strange I had to fact-check, and it appears you're right, there does seem to be a measurable association [0].
Perhaps even more interesting, that same article reports: '...the strongest handedness nding for both sexes was a marked tendency for participants who described themselves as ambidextrous also to describe themselves as bisexual'. How weird.
The mechanism is thought to be a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. Biochemical evidence for this hypothesis was identified in 2017, finding mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons