Romans 16:16 Salute one another with an holy kiss. The churches of Christ salute you.
When was this done?
Commentators offer two perspectives: 1) that this was a greeting performed in coordination with the liturgical rites of the church (i.e., the Lord's Supper); 2) it was simply a standard greeting performed at normal greeting times. Favoring the latter is the point that in the Jewish and Greco-Roman world, a kiss was used as "a form of social respect" used to greet other parties. [Dunn, Romans commentary, 898; Black, 212].
Were there restrictions on who could do this?
In other words, was it only "boy with boy" and "girl with girl"? What little data we have suggests that it was not. The fourth century Apostolic Constitutions indicates a restriction to the practice, to only the same gender; this may indicate that prior practice was different -- or that it had been abused.
A hint may come from the Hellenistic-Jewish document Joseph and Aseneth [Collins, 1 Cor. commentary, 612] in which Joseph refuses to kiss the priestess Aseneth in greeting while she is still a pagan, but once she becomes a Jew he does so. If this is any model, then the destruction of social distinctions ("neither male nor female"), and the purity rendered onto each believer by fellowship with Christ, suggests that the greeting was exchanged between all.
So I can go up to pastor on Sunday and plant a big one, right?
I include this only because there is that critic here and there who says that this naturally follows, but the changes in the social world -- matters of honor; our obsession with the sexual -- mean that translations that render this, i.e., "Have a handshake all around" have the right idea.
-JPH
A reader has added some notes of interest:
Just thought you might find it interesting that even today, in many non-western cultures that do not share "our obsession with the sexual", greeting one another with a kiss is still a common practice. So for example, throughout Russia, Greece, Romania, and the Middle East, it is quite common to see Orthodox Christians greet each other with a kiss - not a big smacker on the lips, mind you, but a light kiss on each cheek. As you say, it's merely the equivalent of a handshake - nothing sexual about it.
Likewise, throughout many regions of Africa, it is very common for people of the same sex (both Christians and non-Christians) to hold hands. Again, nothing sexual about it, just a way of showing friendship. It's really a shame that people in this country are so obsessed with sex that they assume that every other culture in the world (and throughout history) is as well.
I also wanted to mention, regarding the perspective "that this was a greeting performed in coordination with the liturgical rites of the church (i.e., the Lord's Supper)", that, at least in the Orthodox Church, this is still a part of the Liturgy. In the ancient Church, the "Kiss of Peace" took place among all the faithful of the congregation. Again, it consisted of a light kiss on each cheek. In the Orthodox Church today, the Kiss of Peace only occurs between the clergy during the Liturgy in 99% of the churches, although there are a few that still practice it throughout the entire congregation.