the practice of kissing under the mistletoe itself is not pagan but the Celts did revere the plant as having life-giving, protective and fertility properties.
The only source we have attesting to the importance of mistletoe, including ideas of fertility, among the Celts--or more accurately, the Gauls--comes from a secondhand source, Pliny's Natural History (specifically, Chapter 95 of Book 16). He puts forth this claim:
"Upon this occasion we must not omit to mention the admiration that is lavished upon this plant by the Gauls. The Druids—for that is the name they give to their magicians1— held nothing more sacred than the mistletoe and the tree that bears it, supposing always that tree to be the robur... It is the belief with them that the mistletoe, taken in drink, will impart fecundity to all animals that are barren, and that it is an antidote for all poisons. Such are the religious feelings which we find entertained towards trifling objects among nearly all nations."
So this does mention the Druids held it in esteem and that it imparted fecundity to animals. This is secondhand information so we must be a little careful with it, but what is especially pertinent for our purposes is the fact he ascribes this to the
Gauls. The Gauls resided in and around what is modern day France. There are various issues with this supposed connection, but a major one is that the kissing tradition began in
England, not France, at least as far as I can gather.
this may get trickled down into hybrid folklore that can eventually just look like something harmless like kissing under the mistletoe. it may be that the practice of kissing is directly related to a water downed version of fertility or that adorning it over doorways is the pagan value that protects the household and kissing under it then is adopted as a reflection of this value.
Notice the usage of "may". That is, "it may be that the practice of kissing is directly related to a water downed version of fertility or that adorning it over doorways is the pagan value that protects the household and kissing under it then is adopted as a reflection of this value." That's because there isn't any evidence or plausible connection between the two.
There is a connection between this pagan value of the plant and modern-day practice of kissing under it, even if only because it retained a revered position and kissing under it became as a practice more serendipitously than as a direct connection to fertility or protection.
Before you admitted may... but here you claim that
is a connection. But again, none can really be seen.
The argument essentially is that because around the first century AD (when Pliny wrote) the Gauls held the mistletoe in high esteem in religious ceremonies that went completely extinct, somehow that (without leaving apparently a single trace in between) led to a dramatically different practice of kissing under it long, long afterwards. And in another country at that!
The fact that Gauls may have had some affection for the plant does not mean that something significantly later involving it in a different context and a different country for that matter has any connection. It's like saying that because the Fifth Labor of Hercules involved cleaning stables (with cows), and that footballs (in American football) are made from leather, which comes from cows, that the sport of football is derived from the Fifth Labor of Hercules.
I'm not saying however don't kiss under the mistletoe because it's pagan. value for mistletoe has pagan roots but these values are stripped away and the net values that remain tend to be wholesome and easily redeemable in Christian living. What is also important is no one jumps to the pagan values in these practices so as a whole society is not influenced by these pagan values that may have once been there. Valuing the practice wouldn't cause the weak to stumble, because there is no "weak" demographic here that can have a negative effect of the pagan values. There definitely are "the weak" broadly speaking but not as it relates to these pagan values. maybe there was a few hundred years ago but today they are no more. with the example of mistletoe kissing under it has no power and there no weak demographic that would be negatively impacted by this practice.
This is the same with most if not all Christmas values like a Christmas tree, garland, stockings, Santa Clause, etc.. there simply is no weak demographic that these practices would cause them to stumble. Sure many may wag their finger but this is not the same as stumbling. Stumbling is when a person cannot separate the pagan value from the practice or object to such a point they see the pagan power overshadowing them. Someone may bring a directly cursed object by some modern-day witch in their home and feel an overpowering presence from this object. the object is nothing, it has no power, the witch is nothing, they have no power, the power comes from our ignorance and weakness in how we approach these things. We assign power to it when we believe it has power, that's the weakness. No one over the age of 10 actually thinks Santa Clause has any power, or that kissing under the mistletoe has any sort of innate magic. Society has reduced them to fairy tales and good luck charms. obsession with such things is not healthy but it is also considered unusual behaviour in society as a whole that is spoken against so there already is responsible boundaries in place innate in society which can be attributed to Judeo-Christian influence that has over powered these pagan systems.
While I'd largely agree with this, I still contend (as noted above) that few if any of these things can be traced back to paganism anyway.