Here, in a nutshell, is how to eliminate homosexuality and abortion, if you were so inclined.
Homosexuality: It is suggested that one's being gay is influenced by genetics. Thus, the only way a "gay" gene could be passed along is if a carrier were forced into a heterosexual relationship and hence reproduced. Therefore, allowing gays to marry each other (and adopt) would seem to be a great way to gradually winnow down the gay population.
Abortion: People with deeply-held convictions who oppose abortion are going about it the wrong way. We get a lot of our social, religious and political views from our parents and therefore the best way to winnow down the number of people who support abortion is to let those who currently support it go right ahead and do it. Presto: In 100 years hardly anyone will support it - or for that matter, I suspect, be a Democrat.