It is possible that the child is being raised by same-sex parents; but, highly unlikely. It has been proven scientifically that the brains of male homosexuals are structured differently than the brains of hetersexual males. The same is probably true for lesbians; but, there have been no studies done on the brains of female homosexuals.
There are many heterosexual couples who have gay or lesbian children. Many of these individuals know from a very early age that they are gay or lesbian. This little boy may be the gay son of a heterosexual couple who realizes his sexual orientation at an early age.
As for the child who is confused - it's time for a talk on the birds and the bees and homosexuality in the animal kingdom (human or otherwise). Explain that the other boy is either gay; or, for some reason is exploring his sexuality at an early age. This does not mean that he has to participate. If he doesn't want to kiss the other boy, there are gentle ways of saying "no". You might want to go over a few things he can say to this boy if the incident repeats itself.
Both boys require gentle, compassionate help at this time. This can be a learning experience that will empower both boys or it can become a nightmare.