I knpw that 23andme has done a Gwas of sexual orientation and they couldn't find a genetic association for male or female homosexuality. I guess individual prenetal hormonal and epigenetic processes are the major reason why some people are gay. But I have still the opinion that there is indeed a unknown genetic reason why some people are gay.

 

Can we say when a gwas was unsucessfull in finding a genetic association for a phenotype there isn't a genetic background existing?

 

 

They also have done two linkage analysis for male homosexuality. If the major cause for homosexuality is prental hormonal/epigenetic and male homosexuality is very common in the population, so those two linkage analysis are unworthy?

No, you cannot say that there is no genetic background. Rather it is likely that it is multi-factorial so that a simple GWAS won't be able to identify associated genes.