Ill list the ways.
1. It can bring huge financial and emotional stress.
Homosexuals can sue people who are exercising their religious beliefs. For example, a heterosexual married couple with children who do not want to rent a room in their own family household to homosexuals could be sued for discrimination based on "sexual orientation." This can incur significant financial and emotional stress upon the family, not to mention the "prior restraint" effect of the fear of being sued, which results in a family not renting out a room and losing the income. In addition, consider the 2015 Colorado case involving a baker who refused to bake a wedding cake for a gay couple's wedding on religious grounds. The subsequent fine of $135,000, upheld by the Colorado Supreme Court, bankrupted the baker. Was that not harmful? Of course it was, not only to the baker and his family, but to countless others who were denied the baker's services.
2. The health risks are enormous to themselves and others.
The fact is that homosexuals do not live as long as heterosexuals due to the health risks associated with the lifestyle, and billions of dollars are spent annually in health care for them. See Statistics on HIV/AIDS and health related issues
But the HIV/AIDS epidemic is not only in the homosexual community. It has crossed over to the heterosexual community.
Whether or not you want to say that HIV/AIDS is a homosexual disease, the fact is that it is highly prevalent among the gay and lesbian community due to their great number of sex partners. The collateral damage to the rest of society, as far as health risks, cannot be denied.
3. Gay Marriage means having the morals of the minority forced upon the majority.
This can also be said in the reverse. Either way, there is a problem. Normally, morals should not be forced on anyone, though there are exceptions. We force morals on others by preventing them from stealing,
"A recent CDC study found that in 2008 one in five (19%) MSM in 21 major US cities were infected with HIV, and nearly half (44%) were unaware of their infection." (http://www.cdc.gov/hiv/topics/msm/index.htm)
25% of HIV infected in U. K. unaware of their infection: "Of the estimated 86,500 people living with HIV in the United Kingdom, about 25 percent are not aware they are infected.