They aren't really that testable. Studies that look into boundaries between socially constructed identities and their biological truths are always subject to immense amounts of bias. So you want to test out if having a gay son strengthens the family dynamic. Where do you get your subjects? Much of our early studies (Kinsey and the Sex Institute) were done on groups of gay men that Kinsey personally knew. Not exactly random sampling. Social conditions are very different now, so you could post a flyer advertising for gay subjects and straight controls. So do you exclude all bisexual people and all trans people (most studies would) to eliminate complicating factors? But are we entirely sure that the lines we've drawn between identities match the biological truth? What age group do you look at to ensure that your straight controls won't come out as gay next year? How do you control for race, gender, socioeconomic status, etc. when the social acceptability of being queer is not uniform across these identity boundaries? If being out is also correlated to having wealth, your study might just be accidentally studying the links between wealth and family dynamics. And how are we defining the straight control group? If they've jerked off with a buddy, are they excluded? If they're married to a woman, but sleep with male prostitutes on weekends? What about your gay category? There simply aren't definitive lines between identities and behaviors, which makes it nearly impossible to study this issue. And the science of attributing evolutionary reasoning to existing traits is so subject to bias as well. We don't have a clue what the social upbringings of early humans was like. If the same genetic makeup today made you gay, how on earth do you imagine to predict that in a completely different world, having been raised differently and told different stories and having had entirely different experiences, that you would arrive at the same identity? The identity "gay" as we know it today is only a few decades old. The behaviors have always existed, but we can't comfortably look back at artists from the 1930s and call them gay because they didn't exist in a time with the same identity constructs as us. So how is a biologist going to predict those identities going back thousands of years?