I don't think it would make much difference. Some historians actually believe that James Buchanan our 15th president was a homosexual. Of course back then you never openly discussed your sexual orientation, especially if you were gay. However Buchanan was despised for his poor leadership rather than his sexual preference so I guess it just goes to show that if you make poor decisions as president, people are going to be more critical of your politics than your sexual preferences.
you'd be suprised how many important people such as authors were homosexual, yet the fact that they were doesn't overshadow the fact that they did great things. Are there going to be hateful idiots that would immediately disregard anything the president said? Sure, but they're in a minority. A gay president would be no different than any other president, and his orientation would not overshadow the great things, or the horrible things he/she did in office.