"The husband and wife really love each other."
Translation:Муж и жена очень любят друг друга.
In друг дру́га, the second word is actually masculine in the accusative case. But they don't imply that the speaker is masculine, they're used regardless of the gender.
This expression derives from an older meaning of «друг», 'other, another' (in modern language we use the word «друго́й» for this meaning), and it's in fact not too remote from the English 'each other' (well, it's more like 'other the other' or something like this, but you get the idea).
Although this «друг» is etymologically related to the «друг» meaning 'friend' (because 'friend' is 'the other person near you'), it's not the same word and it doesn't make much sense translating «друг дру́га» as 'friend friend'.
In «друг дру́га», the first part always remains the same (it's originally a nominative; although now it doesn't, of course, represent the grammatical subject of the sentence), and the second «друг» is declined and can be in any case except nominative:
- Они́ чита́ли кни́ги друг дру́га. They read each other's books. (genitive)
- И́нна и Софи́я доверя́ют друг дру́гу. Inna and Sofia trust each other. (dative)
- Они́ зна́ют друг дру́га с де́тства. 'They have known each other from childhood.' (accusative)
- Партнёры дово́льны друг дру́гом. The partners are pleased with each other. (instrumental)
You can even put a pronoun between them:
- Серге́й и Ди́ма зна́ют друг о дру́ге всё. Sergei and Dima know everything about each other. (prepositional, with the preposition о)
- На́стя и Ле́на гото́вы постоя́ть друг за дру́га. Nastya and Lena are ready to stand up for each other. (accusative, with the preposition за)
However, «друг другу» is never plural:
- Мои сёстры и друзья́ хорошо́ зна́ют друг дру́га. My sisters and friends know each other well. (although сёстры and друзья́ both are plural, друг дру́га remains singular)
Well, it's correct because it follows Russian grammar rules. Maybe you could describe your difficulties? Then it would be easier to explain. Here's a word-for-word gloss, maybe it helps:
- муж = [the] husband,
- и = and,
- жена́ = [the] wife,
- о́чень = very much, really,
- лю́бят = love,
- друг дру́га = each other.
«По-настоящему любят» is used much less often than ‘really love’.
But I agree that all the variants you’ve mentioned should be accepted.