Here, some means
some
1. being an undetermined or unspecified one
So there isn't an issue regarding plurality: justification can be singular.
You can use justification as a plural like you would with good reason.
Here is an example in the same sense as your example. I don't know why there were budget cuts. There must be some good reason for the recent budget cuts →
- There must be some justification for the recent budget cuts.
There can also be multiple undetermined justifications. I don't know why there were budget cuts. There must be some good reasons for the recent budget cuts →
- There must be some justifications for the recent budget cuts.
If there is just one justification for a particular action, then justification (singular) is correct.
- We have just one justification for the recent budget cuts.
If there are more than one reasons for a particular action, then justifications (plural) is correct. For example, imagine that there are ten reasons for the budget cuts, but only three are presented: here are some reasons for the recent budget cuts→
- Here are some justifications for the recent budget cuts.
Here's a headline I found online that uses justifications.
18 Solid Justifications for Cloud Computing -- and 10 Situations Where It Doesn't Work
This is one of those strange exceptions to the rule in English - it's not correct, but it's well understood by native speakers.
Normally, you would expect the plural to be used - multiple weddings for multiple couples - but it doesn't scan as well when read/spoken. This occasionally results in conflicting plural/singular for a handful of phrases where it 'feels' nicer to say it in a particular way.
However, in the example, while it refers to multiple brides and grooms, it refers to their singular wedding day. Readers would interpret this sentence to understand multiple couples and their respective single day.
Strictly, the singular day could be shared by all the people or could be a different day for each person. However in this sentence one considers each person as an individual because it's very unusual to have a shared group wedding ceremony.
It may also be inferring some brides and some grooms, in various combinations. The unfortunate thing about this particular example is that it requires some awareness of recent history in context of marriage practice.
In a pure grammatical sense, the start of the sentence is wrong. For clarity, I would write,
In the seventies, the bride and groom often stayed apart on their
wedding day before the ceremony.
The use of "brides and grooms" in the example implies that most/all unmarried couples behaved in this manner, and readers would understand this given context. While it's not grammatically correct, it's one of those evolving aspects of language we can't really stop people from using!
Best Answer
I can see why you are wondering.
An interaction necessarily implies at least two persons, animals, events, etc.
There were two parties, but a single pair during a single period for a single purpose. So it is viewed as a single interaction.
Here we use a plural because, despite the fact that we are discussing a single period and a single pair, we are contrasting opposite types of interaction.
In other words, whether singular or plural is used depends in large part on whether similarities or differences are the topic.