The issue for most homosexual couple is not a semantic issue: to be allowed to have their relationship legalised by calling it "marriage", but to a legal issue: to be allowed to legalise their relationship for the reason that they want to have the same legal rights.
Here in Europe, in the countries which have the system, it is referred to as "civil partnership" and the LGBT community is happy with that despite it not being called "marriage". Actually, complaints about the system in the UK came from heterosexual couples who want want to have access to civil partnerships instead of marriage (ref: http://www.bbc.co.uk/news/magazine-11625835).
An example to the rights that are in question is that health insurance in Germany is cheaper for couples who marry or who are civil partners than it is for two adults in a relationship not legally defined (i.e. two girlfriends, a girlfriend and a boyfriend, two boyfriends).
Finally, by linking homosexuality and incest, the poster tries to create a link between homosexuality which is now acceptable by most people to incest which is universally unaccepted and immoral. In western countries one is also legal and the other isn't.