Most parents I think are willing to take their children 'as they come', loving them whatever but there has always been some who for their own selfish reasons will not accept what nature sends, before prenatal tests these parents have sent their unacceptable children away or even killed them as the Trojans did when they left handicapped and weakling children exposed on mountains. Female babies in some countries are still killed, a boy child wouldn't even if a test showed they were liable to be gay, they'd simply be taught not to show their true nature thus causing untold damage to them.
I think you may be engaging in wishful thinking. Since prenatal testing for Down Syndrome began, the incidence of children born with it has plunged.
http://en.wikipedia.org/wiki/Down_syndrome#Abortion_rates
A 2002 literature review of elective abortion rates found that 91–93% of pregnancies in the United Kingdom and Europe with a diagnosis of Down syndrome were terminated.[51] Data from the National Down Syndrome Cytogenetic Register in the United Kingdom indicates that from 1989 to 2006 the proportion of women choosing to terminate a pregnancy following prenatal diagnosis of Down Syndrome has remained constant at around 92%.[52][53]
It would appear that in reality, the overwhelming majority of parents who become aware that their child will be born with Down Syndrome choose to abort. The mental exercise I propose is whether or not it would be OK for parents to make similar decisions regarding a child being born with a predisposition to be gay. I think supposing that parents would not care because they don't care or would simply teach their children to hide their nature is a bit of a stretch; because clearly they do not do so with regard to children with Down Syndrome. If parents detect Down Syndrome in unborn children, they overwhelmingly choose to abort. So what would they do if they could detect homosexuality? And would it be OK?