My first inclination after reading this question was how would I even know my colleague was homosexual? Many feminine men are straight and a LOT of gay men are not effeminate at all. Unless someone from my organization "warned" me, which would arouse feelings of discomfort toward that person, I generally have no way of knowing unless they volunteer it but such a conversation as that would not even be initiated unless we had already established some level of comfort in each other company.
Someone who is homosexual is very capable of being as good a colleague (reliable, hard-working, amiable) as someone that is heterosexual. When assessing a colleague, its important to judge them the characteristics relevant to them being a good colleague and by the standard we wish others to judge us by as well.