The idea that homosexuality doesn't "harm anyone" is only partially true. Perhaps homo/bisexual couples are not immediately damaging their partner like rape or murder would. However, man-on-man intercourse in particular has significant risks; research has shown the fragile anus is not meant to be penetrated, and there are even more imminent risks to fecal exposure.
As stated by Centers For Disease Control and Prevention: "In 2008, men who have sex with men (MSM) accounted for 63% of primary and secondary syphilis cases in the United States. MSM often are diagnosed with other bacterial STDs, including chlamydia and gonorrhea infections. Gay and bisexual men can be infected with HPV (Human Papillomavirus), the most common STD in the United States. Some types of HPV cause genital and anal warts and some can lead to the development of anal and oral cancer. Men who have sex with men are 17 times more likely to develop anal cancer than heterosexual men."
"Men who have sex with men (MSM) accounted for 63% of the estimated new HIV infections in 2010… For gay and bisexual men, HIV, hepatitis, and other STDs are of particular concern… The rate of new HIV diagnoses among MSM is more than 44 times that of other men, while the rate of primary and secondary syphilis among MSM is more than 46 times that of other men." (Emphasis mine)
There is also the fact that it is mentally harmful, as both lesbians and gays tend to have higher stress levels and periods of depression than heterosexuals. Lesbians also tend to have a higher rate of obesity and smoking (indirect results of a lesbian lifestyle), which increase health risks.
Obviously, it does harm the individuals involved, so you will have to pick a different argument besides that. The burden of proof is on you to support the idea that homosexuality is "okay."
Perhaps I would accept that "being homosexual is OK" if you were to present me with a logical argument explaining why homosexuality is acceptable. Simply stating that it is "OK" is not an argument.
The burden of proof is still on you. I'm still waiting for you to present a logical argument to support you claim that "homosexuality is OK."