Although Buddhist lay people might be interested in having a monk perform their marriage ceremony, it is not allowed and doing so is classed as a heavy rule (Saṅghādisesa). Such heavy rules have penalties and require the monk to lose his full status for a minimum of 6 nights1mānatta and additional probation time if concealing the offense occurs.
Should any bhikkhu engage in conveying a man’s intentions to a woman or a woman’s intentions to a man, proposing marriage or paramourage — even if only for a momentary liaison — it entails initial and subsequent meetings of the Community.2sañcarittasikkhāpadaṃ pana bhikkhu sañcarittaṃ samāpajjeyya itthiyā vā purisamatiṃ purisassa vā itthimatiṃ, jāyattane vā jārattane vā, antamaso taṅkhaṇikāyapi, saṅghādiseso.5.
Effort. The Commentary says that to “engage in conveying” means to take on the role of a go-between. This includes helping to arrange not only marriages and affairs, but also “momentary associations” that, from the way it describes them, could include anything from appointments with a prostitute to arrangements for X to be Y’s date.
The Vibhanga sets the component factors of a go-between’s role at three:
1) Accepting the request of one party to convey a proposal;Saṅghādisesa 5 (BMC 1)
2) Inquiring, i.e., informing the second party and learning his/her/their reaction; and
3) Reporting what one has learned to the first party.
If you can recall what happens at a typical marriage ceremony, the one who leads the ceremony asks the questions to each party just as written in the factors of the rule as quoted above from The Buddhist Monastic Code. This is why you have probably never seen bhikkhus or “bhikkhunis” performing marriages. If you do see this, take note that it is really bad. Doing so entails the same type of punishment for a monk merely fondling a woman’s body or even holding her hand with lustful intent.3Bhikkhunis are defeated if they consent to being touched below the collarbone and above the knees.
Why do we have this rule?
Like more than half of the marriages in America that are doomed for failure, so too in the Buddha’s time, a monk named Venerable Udāyī arranged a marriage that failed miserably. The monk was blamed, and the Buddha created a heavy rule for this. At that time, and also in some cultures existing today, marriage is a serious thing that affects the woman (and man) for life. It should not be taken lightly. Because of this, we now have this rule.
What Can Monks Do?
At best, a monk can chant at a marriage, give a dhamma talk or both after the marriage ceremony has finished. If you see monks at a wedding, merely chanting or giving a dhamma talk is usually what they are doing there.