First of all, this oft-repeated "rule" makes no sense. An author cannot know for sure whether his readers already know about some topic. So a more nuanced statement (it is not a rule but a guideline, and an oversimplified guideline at that) is:
"the" is put before a noun or a noun phrase to indicate information that the writer assumes is known to the reader.
See the second sentence in the Wikipedia article on English articles, which is cast similarly to mine (Use of the definite article implies that the speaker assumes the listener knows the identity of the noun's referent (because it is obvious, because it is common knowledge, or because it was mentioned in the same sentence or an earlier sentence)).
However, even this guideline does not describe actual usage very well, and there are many times when English diverts from this "rule." For a fuller, if imperfect, discussion of this, see my answer to Are there any simple rules for choosing the definite vs. indefinite (vs. none) article?. In fact, the usage I describe in this answer does not correspond to this guideline.
As for your two sentences, note that both
love and
marriage are
abstract nouns, which are often described as nouns that express things that we cannot touch, taste, see, hear, or smell. This is in contrast to
concrete nouns such as
dirt,
smoke and
noise, which are objects that one can taste, touch, see, hear, or smell. However, even as I write this, I am not sure this description precisely classifies such nouns as
music. Certainly, we can talk about
music in the abstract even though we can hear it. I guess music we can hear is a specific instance. The same can be said for
language.
When an abstract noun is used as a general concept, we do not use the definite article.
Love is everywhere.
War is everywhere.
Sadness is everywhere.
Happiness is everywhere.
Marriage is everywhere.
Divorce is everywhere.
Language is everywhere.
(Note, of course, that concrete nouns can also be "everywhere": Dust is everywhere.)
When we want to talk about a specific instance of this abstract notion or concept, we use the definite article:
The love between people is the most beautiful in the world.
This contrasts with, say, the love between elephants, which is another specific instance of love.
Note that the use of the in both noun phrases, the love between people and the love between elephants is not determined by whether the author assumes his reader knows about such instances of love. Put another way, the definite article can be used to introduce new information. Additionally, the use of the is not determined by the fact that this is the second mention of love between people. One could finish this paragraph using the last sentence with the zero article, and it would conform to the usage described in this answer, namely that it would be a reference to love as a general concept.
Now let's look at the middle sentence of your example:
There are love between people, love of nature, and love of knowledge.
Here, there is no article before love. Or, many would say, there is the zero article before love. At any rate, all three examples here consider love as an abstract noun.
We could use the in each case, and thus give three specific instances of love:
There are the love between people, the love of nature, and the love of knowledge.
Note that love remains an abstract noun with the; it is just that with the, we are talking about three specific instances or cases or examples of it.
Moreover/furthermore (they mean the same), we can also use the indefinite article with abstract nouns; when we do, it means a type of or a kind of. This is similar to how we can use the indefinite article before mass nouns to talk about a kind of something, such as a kind of coffee or a type of cheese. So
There are a love between people, a love of nature, and a love of knowledge.
expresses three different types or kinds of love. So, with this sentence we have seen three different uses of an abstract noun, and the role that the article (definite, zero, or indefinite) plays in these uses.
Note that even though I have written (or typed) there are, I greatly prefer there is, especially in the example with the indefinite article. This is because the first item in each list is singular. See the accepted answer to the question “There Is”/“There are” depends on plurality of the first list element or not?, and thanks to CopperKettle for the comment.
Marriage is also an abstract noun.
Gay marriage is an abstract noun phrase. But it works pretty much the same way as marriage by itself.
Thus, your first sentence talks about gay marriage as a general concept. And this is why it is correct not to use the in your last sentence (which I have corrected by replacing 'against' with 'opposed'):
Even so, gay marriage is still strongly opposed by many countries.
This means that many countries still oppose the general concept of gay marriage.
The sentence
Even so, the gay marriage is still strongly opposed by many countries.
is an example of a specific instance of the general concept, and it might be talking about the gay marriage of two specific individuals, such as the gay marriage of Prime Minister Smith and Home Secretary Jones.
Hopefully this answer has explained the usage of the italicized noun phrase in the following sentence of yours:
As we all know, gay or lesbian love is never a new topic.
That this usage deviates from the "rule" or guideline which we started with cannot be more plain: The author takes for granted (i.e., assumes) that "we all know" something about gay and lesbian love, but this assumption does not mean that the author uses the. In fact, he deliberately does not use the because of the usage described in this answer.
