Don't attack other peoples' beliefs just because you don't believe it doesn't mean it is stupid to believe it.
I'm quite happy to viciously attack logically flawed, hateful, divisive views that cause harm and encourage others to do as much as possible. As per brenden, ninatron et al, such oppressive beliefs are stupid and should be attacked.
Obviously evidence for the genetic argument would be that homosexual people have certain traits at young ages - boys being feminine for example.
That is far from obvious genetic evidence and the term 'homosexual traits' is stepping into pseudoscientific territory. What is a homosexual trait in a person without any sex drive or sexual attraction?
A significant number of homosexual males consistently demonstrate unremarkable mannerisms devoid of obvious feminine characteristics from childhood onwards. Many are completely typical.
In any case, this is the territory of gender role identification and cultural experience, not sexual orientation - which is a demonstrably separate phenomena, though often correlated with the latter. Mannerisms and gendered behaviours often change over time and setting. You put a young male around hypermasculine adult males that he relates to and identifies with and his mannerisms will become similarly hypermasculine. Sexual orientation appears somewhat independent of this, though a factor that sometimes influences other gender-related (e.g. having a particular sexual orientation may make it difficult to identify with adult role models of the same sex, but easier to identify with those of the opposite sex - thus, more like to adopt mannerisms of the latter).
I completely agree however about the combination. There is a almost certainly a complex interaction between genotype, phenotype, gender ID, socialisation etc.