Hm, first of all, if they start kissing each other with passion, I seriously have to ask if children should see a movie of that kind at all, no matter if the passionate kissing (which is most likely to be "foreplay" in most movies) is between a man and a woman or two men or two women.
Nevertheless, asking a question like that is problematic as it includes the idea of kissing people of the same gender is unnatural - in my opinion, that is the wrong way to raise children nowadays.
Therefore, let me ask you another question: Why should the explanation be another than the one you'd give if your children would see a "common" couple kissing?
But no more talk, here's my explanation:
"If a person loves another person, it is not unusual for them to kiss each other. Your mommy and daddy are kissing each other, because they love each other and just like that those two men were kissing each other because they love each other. Kissing is an expression of sympathy."
Yet I have to say I will never, under no circumstances, be so insane to condemn children to live in this world.