Is getting married a Mitzva in its own right or a Hechsher Mitzvah (i.e. necessary precondition) for another Mitzva?
Mitsva is Piria Verivia , and for male only (according to the Halacha, but some Tanayim think that female too are concerned by this mitsva) @DoubleAA taught me that an additional mitsva to getting married with Kiddushin and Ketuba. An additional negative mitsva to not be in intercourse with a woman without Kiddushin and Ketuba, in Mishna and Gemara we can study details about know the kinyanim and the amount of the Ketuba, the Get. All this is Mideorayta.
For source see Sefer Hamitsvot from Rambam and Sefer Hachinuch (But you can study it Mikraot and Chazal), the follow numbers and links are from Sefer Hachinuch:
Reproduction, --> 1
The act of getting married --> 552
Not to enter in sexual relationship with a woman without Ketuba and kiddushin, --> 570
But if we read the Chinuch, one statement should be thinking about:
ועובר על זה ונשא אשה מבלי שיקדשנה תחילה ביטל עשה זה.
Whom married a woman without Kiddushin before cancels this positive commandment.
I really wondering, why not speaking about a man do not marry at all? Perhaps because there is no Bitul "beshav Veal Taasse".
As Nafka Mina
We can note that this mitsva applies when old people marry. P.e. An old widower and an old widow, they want to live together, but the couple will not have children. They are duty to rules of kidushin and Ketuba as all young couple.
Similar question (and answer):
Perhaps, we can suppose, but I have no proof, that this mitsva is a modaliy. I.E., a way to make something. If a man wants to marry a woman, so the right way is to start with Kiddushin and Ketuba. But if he does not want, no problem.
I have a partial proof that it is not the case. Rambam in Sefer Hamitsvot Mitsva 96 counted "Uncleanness of eight Shratsim", he explain a general rule for its count. A mitsva regarding uncleanness does not mean that we need to be unclean by this uncleanness or that we are prohibited from this uncleanness. The translation of the mitsva is. Whom touch this thing will be unclean as this and this; if this thing touched someone this uncleanness will happened, etc.
Ramban does not agree and see rules of cleanness-uncleanness as details for mitsvot involved in cleanness-uncleanness. It is similar to the rule to not give Korban with default, the knowledge regarding defaults in not Mitsva but details of this prohibition. Ramban includes this knowledge to the VII principle (not to count details of mitsvot as separated mitsvot).
Regarding Kiddushin, we can imagine that Tora prohibits many kinds of relationship, see mitsva 570, and the way to avoid this prohibition is kiddushin and ketuba. So, if it was the case, Ramban was not agreeing. But Ramban seems to agree. So definitely this is a mitsva.