Identical male twins are more likely to both be gay than fraternal male twins. Looking beyond twins to families, men and women share something here, but with radically different implications. Being gay does seem to run in the blood, but on the mother's side only. The evidence suggests something on the 'X' chromosome, which, you'll recall from junior high biology, means it's part of mom's genetic contribution. What's more, men tend to identify as straight or gay and stick to it most of their lives.
Bisexuality is far less common for men than for women.
Different for girls For women, though, the family picture's both different and the same. But nearly a third of the time, when it's true of the daughter, it's that way for mom as well. As many know from their own experience, it's often the daughter who comes out first. Maybe lesbian moms somehow teach their daughters to follow their hearts, even when they don't take their own advice. If culture didn't tend to discourage men's inner lives, then maybe we would be more attuned to those soft urgings drawing us to the more fluid sexual identity women enjoy.