This is a huge understatement. In my experience (which goes back to the early 60s) a very small percentage of gay/bisexual men are out, and there are vast numbers of ostensibly-straight men who have gay sex on the side. And in most cases their families, friends and coworkers are perpetually in the dark.
(Perhaps a lesbian will give us some info on her perspective.)
Surprisingly, that’s very often not the case. Families who have already struggled to accept one “black sheep” often put more pressure on other family members to be heterosexual. If you have only two sons or daughters and one is gay, there’s lots of pressure to “pass on the family name” or to “walk Daddy’s little girl down the aisle.”
This has been alluded to already, but I wanted to emphasize that there’s no particular reason to believe that the same factors are responsible for homosexuality in both men and women. The gene(s) that contribute to a man being attracted to other men could be completely different from the gene(s) that contribute to a woman being attracted to other women. If this is the case then the genetic component of homosexuality could be inherited from a heterosexual (or bisexual) parent of the opposite sex.
Let’s imagine for instance that there’s a gene that predisposes people to be turned on by male pheromones. In a woman such a gene would, from a purely reproductive standpoint, be either neutral (if it doesn’t make her any more interested in sex with men than the average straight woman) or an advantage (if it makes her want to have sex with men more often than the average straight woman). Daughters who inherit this gene would be in the same situation as their mother, and would be at least as likely to have children as women who did not have this gene. A son who inherited this gene would find himself sexually attracted to other men, which is likely to hurt his reproductive odds. Yet even if this son is totally homosexual and never serves as a sperm donor or anything, the gene that contributed to his same-sex attraction would continue to be passed down the female line in his family.