There are two parts of a kiss:  The intent and the act itself.  If both of these are pure, then a kiss is not sinful.  If a kiss either has an impure intent or the act itself is impure then the kiss is sinful.

By being pure in intent, I mean, a kiss that is meant to show philial affection.  If you bring erotic feeling into your kiss, your kiss will be erotic, and that is only proper between a married couple.

Now, as for the act itself, not all kisses are created equal.  Some kisses  sexually excite the other person.  This would be considered wrong to do between gay couples since it risks leading your partner into a sinful activity.

However, a chaste kiss can reveal a philial passion that is in itself good.  That can be no great crime.