We have several different uses of marry and married in play. Some of the meanings deal with an act and others with a status.
There are two basic uses of marry, both relating to the act:
[transitive or intransitive] to become the legally accepted husband or wife of someone in an official or religious ceremony:
[transitive] to perform the ceremony of marriage as a priest or official:
(most of the additional uses you will find in a dictionary, as to acquire something through marriage as in marry money, or to find a spouse, as in marry her daughter to the prince, derive from the first).
There are two non-overlapping senses of married, the first referring to the status, the second referring to the act:
having a wife or husband
to begin a legal relationship with someone as their husband or wife
The act of becoming a husband or wife (i.e. taking vows, participating in a wedding ceremony, signing legal papers, et al), represented by the first meaning of marry and the second meaning of married, is usually expressed as getting married. Thus, a native speaker would be more likely to say
I am getting married in July.
I will get married in July.
Congratulate me, I'm getting married.
Congratulate me, I'm going to get married.
You can say
I am to be married in July.
but like all I am to V statements (I am to be named chief, I am to take the pills daily), it is formal and sounds somewhat stiff.
Usually, to say being married in referring to the act is usually to use the second sense of marry in the passive voice.
Parson Brown will marry us on Saturday. → We are being married by Parson Brown on Saturday.
Otherwise, being married is often in the first sense of married, to be in a state of marriage with someone.
Being married to someone for 20 years takes patience and sacrifice.
SHORT ANSWER:
Neither version expresses two independent thoughts: the participle clause modifies the meaning of the main clause.
LONG ANSWER:
An independent clause requires a finite (tensed) verb. A participle† clause/phrase‡ like this cannot be an independent clause; it must be parsed as subordinate, either an adjunct (modifier) on one of the constituents or a ‘supplement’ to the main clause. The term ‘supplement’ is that of the Cambridge Grammar of the English Language, which gives examples:
i a. His hands gripping the door, he let out a volley of curses.
b. This done, she walked off without another word.
ii a. Realising he no longer had the premier's support, Ed submitted his resignation.
b. Born in Aberdeen, Sue had never been further south than Edinburgh.
The [italicized] non-finites are supplements with the main clause as anchor. Those in
[i] contain a subject, and belong to what is known as the absolute construction, one
which is subordinate in form but with no syntactic link to the main clause. Those in [ii] have no subject, and are syntactically related to the main clause in that the missing subject is controlled by the subject of the main clause: it was Ed who realised he no longer had the premier's support, and Sue who was born in Aberdeen. In neither [i] nor [ii] is there any explicit indication of the semantic relation between the supplement and the anchor. This has to be inferred from the content of the clauses and/or the context. The natural interpretation of the supplement in [ib], for example, is temporal (“when this was done”), and of that in [iia] causal (“”because he realised ... ”). Both constructions allow gerund-participials or past-participials. [1265-66; boldface emphasis mine]
The position of the supplement is significant only if the interpretation is temporal: the supplement and the predicate of the main clause have to be set in their narrative order. But a causal supplement may be set in any position.
There is an additional twist when the gerund-participle is being. The ordinary way of expressing the fact that John was sick when he went to work is to employ the bare adjective, as TRomano suggests. Adding the otherwise redundant participle implies that you have some more complex meaning in mind, and consequently it virtually compels a causal interpretation:
Being sick, John went to work = Because he was sick, John went to work.
John went to work, being sick = John went to work because he was sick.
I don't think that is what you intend!
The version with getting is naturally interpreted as a temporal: John went to work and on the way started to get sick.
† When the -ing form is employed this way it is called a present participle. The term gerund is reserved for situations in which the -ing form is employed as a noun or as the head of a phrase which acts as a noun. If you wish to speak of the form rather than the use you could follow CGEL and call it a gerund-participle, or you could call it simply the -ing form.
‡ Traditional grammar treats constructions like being sick as phrases because they have no overt subject; some modern grammars treat them as clauses because the subject is inferrable from context. It doesn’t really matter what you call them, as long as you and your readers have the same understanding of the terms.
Best Answer
Because this is an absolute clause. It modifies the main clause - the part of the sentence before the comma.
If you used is, it would become a stand-alone, independent clause. You would need either to use a semicolon to divide the two clauses, or just break the sentence outright into two separate sentences:
Thus, it will be wrong only if you change the ing form into the finite-verb form is and make no further changes.
Here's a simple example of an absolute construction:
See: "CopperKettle's answer read" is not explicitly bound to the main clause, but it is logically connected to it - to the whole of it, not to some particular word.
An absolute clause can use a non-finite verb (an ing or ed verb - like "being" or "read"), but not a finite verb like is.
You could break down this example sentence into two sentences if you want to get rid of the absolute clause:
Reference:
Quirk et al., A Comprehensive Grammar of the English Language, Unit 15.58, "Nonfinite and verbless adverbial clauses".
P.S. Concerning your comment question,
No, it's a non-finite adverbial clause. It modifies the verb phrase "scrunched up his brow". And the "understood subject" of this clause is Nazmul: it is identical in reference to the subject of the superordinate clause.
How to distinguish an absolute clause? It usually contains a noun to which the non-finite verb connects. This noun (or noun phrase) is an "overt subject" of the clause. Take, for example, such formulaic absolute constructions as "God willing" (noun: God), "Weather permitting" (noun: weather).
Example of a sentence where one non-finite clause is an adverbial, the other is an absolute construction:
The absolute clause is "every labor sped". The past participle "sped" describes the noun "labor". "Every labor was sped".
The adverbial non-finite clause is "at night returning" - it contains no logical subject for "returning", and is connected to the verb phrase "sits him down the monarch of a shed". Its 'understood subject' is "He".