Before refuting the arguments which try to relate homosexuality with genes, one should consider the study/s conducted by few researchers -those advocating homosexuality:
There are at least more than one studies conducted in this case.
1) Of these only one study, BAILY & PILLARD (1991) included both non-twin biological brothers & adopted (unrelated) brothers in addition to identical & fraternal twins.
2) Identical twins were most likely to both be gay; 52% were in agreement for homosexuality, as compared with 22% of fraternal twins. This result would support a genetic interpretation because identical twins share all of their genes, whereas fraternal twins share only half their genes.
3) Moreover BAILY & PILLARD found that 9% of non-twin brothers of were concordant for homosexuality & 11% of adoptive brothers were both brothers gay.
Rationale (why the research is conducted)
1) Today, people practicing homosexual lifestyles seek tolerance and acceptance from society at large, and have achieved considerable ‘success’ (perhaps not in Malaysia yet). By organizing themselves into a political movement, homosexual vigorously lobby psychiatric, legal and political organizations for changes in discriminatory laws to include sexual preference, and seek such things as the right to adopt children, health, and pension and tax benefits enjoyed by legally married HETEROSEXUAL couples.
2) The purpose important to their (gay & lesbian) political agenda is the idea that homosexual behavior is an “inborn” characteristic, perhaps genetic, like race or gender. Hence according to these homosexual organizations, all moral reservations regarding their behaviors have to be dropped.
 Identical twins develop from the splitting of a single fertilized egg, thus genetically identical. Fraternal twins, on the other hand, develop concurrently from 2 separate eggs fertilized by 2 sperms, therefore genetically similar as any non-twin brothers might be.
 Faris, Donald, The Homosexual Challenge (1993)