First of all, no professional scientist in the last 40 years has even claimed to have found any evidence that queerness is always genetic in all cases. Even the highly publicized identical twin studies by Michael Bailey and Richard Pillard, which were so often touted as evidence for a "gay gene," never found that any more than 52% of the identical twins of queer people were queer. The other 48% of identical twins of queer men were not queer. This leaves at least 48% of the queer community unaccounted for by genetic theories. Is it so hard to imagine that a few of those 48% might want to lay claim to having made a choice?
It's also very possible to argue (although certainly not all queer by choice people would argue this) that even the 52% of identical twins of queer people who were also queer themselves can be accounted for by entirely nonbiological causes. Identical twins are exposed to extremely similar social environments and besides, in a world where queerness is so widely believed to be genetic, if your identical twin comes out to you as queer then that's got to drastically increase your likelihood of intensely questioning your sexual preference, and which in turn must surely increase the likelihood of discovering same-sex attractions which you might never have discovered if your identical twin hadn't caused you to look for them.
For more information on biological studies of queerness, see the following pages: