Incest and equal rights for LGBT are two completely separate issues. Without going into the merits of the issues, there are pretty much no people lobbying for father/daughter marriages, while there are lots of people lobbying for gay marriage. One goal of society is to codify rules that its members believe in, and in order to do that, people have to speak up. So if you want to marry your kids, start lobbying!
(You can already marry your cousin, if that's good enough for you.)
(You can already marry your cousin, if that's good enough for you.)