Is your husband absolutely, positively, sure that these two men are in a homosexual relationship with each other and that was the reason for the kiss? Kissing was common in the family of one man I dated. Men kissed their brothers and their fathers and sons on the lips. In some cultures or families this may be the norm.
I would not suggest confronting them directly at all, unless you already have a relationship with them, and direct knowledge of their sexual relationship. Assumptions are not good enough. Keep in mind too that even if you have this information, you may “come off” as self-righteous and judgmental regardless, just for actually saying something. Generally speaking, people who reject Church teaching know they are rejecting it. Prayers for conversion would be the most appropriate.