This past week I spoke about the mitzvah of kiddushin. In this week's parsha is the mitzvah of peru u'rivu. The question is how does kiddushin fit in? Is this also a mitzvah? There are 2 main opinions in the Rishonim.
The Rambam (Hilchos Ishus 1:2, Sefer HaMitzvos 213) states clearly that kiddushin is a mitzvah. He counts the mitzvah of peru u'rivu and kiddushin as 2 mitzvos. According to the Rambam, if one wants to create a relationship with another person, it is a mitzva to do it through the mechanism called kiddushin.
The Kesef Mishna quotes Rav Avraham ben HaRambam who explains that the Rambam holds kiddushin and nisuin is one mitzvah. Kiddushin is considered the beginning of the mitzvah and nisuin is the end of the mitzvah. The KEsef Mishna is m'dayeik from the way the Rambam lists the mitzvah in his header. He writes "the mitzvah of nisuin through kiddushin and kesuva". I think you can add a diyuk from the sefer hamitzvos where the Rambam writes "it is a mitzvah livol isha through kiddushin". You see the entire process is the mitzvah.
You can contrast this with the Sefer HaChinuch who writes the mitzvah is to be m'kadeish an isha through kesef,shtar, biyah. He makes no mention of nisuin.
The Rosh in Kesuvos (perek 1 siman 12) says kiddushin is not a mitzvah. He says if one wants to be m'kayeim peru u'rivu yuo can do it through a pilegesh, you don't need to get married. In fact it's not even a hechsher mitzvah like shechitah. By shechitah, you can't eat without shechitah, but you don't need kiddushin to be m'kayeim perui u'rivu
1) Birchas Eirusin
How do you understand the nature of the beracha on kiddushin? The Rosh writes it is not a birchas hamitzvah but a birchas hashevach. That is why the loshon of the beracha is not comparable to a regular birchas hamitzvah. We don't say "asher kidishanu l'kadesj isha". Plus we mention the issur arayos. We never find that we mention the issur in a birchas hamitzvah. Finally, that is way the beracha is said after kiddushin and not before, there is no requirement for oveir l'asiyason since it is only a birchas hashevach.
The Rambam (Ishus 3:23) says the beracha must be before the mitzvah due to oveir l'asiyason. In fact if you omit the beracha you can't say it after the kiddushin.
2) Mitzva Bo Yoser M'bishlucho
According to the Rambam we understand why the gemara says it's better to do kiddushin yourself. Since it's a mitzvah it is better to do it yourself. According to the Rosh why is it better? In fact the Netziv (Hemek Shailah Siman 165) says mitzva bo yoser m'bishlucho would apply to a hechsher mitzvah but only a hechsher mitzvah mentioned in the Torah (like Kovod Shabbos). A hechsher mitzvah not mentioned in the Torah does not have this requirement. Since kiddushin isn't mefurash b'kra there is no mitzva bo yoser m'bishlucho by kiddushin if it is only a hechsher mitzvah. (He uses this as a rayah against the Rosh).
You can answer the Rosh with a Ran in Kiddushin (Reish Perek 2). The Ran says mitzva bo yoser m'bishlucho applies to a man because it is assur to marry without seeing the woman. However, mitzva bo yoser m'bishlucho applies to the woman as well since she helps the man fulfill his mitzvah of peru u'rivu. You see the Ran holds like the Rosh that kiddushin is not a mitzvah but mitzva bo yoser m'bishlucho still applies.