1) The Shitah Mekubetzet (7b, beginning of DH Gufa Amar Rav Nachman) writes in the name of the Ge'onim that one requires ten for Birkas Chasanim because it is a Davar sheb'Kedushah.

2) However, the Magen Avraham (OC 128:22) writes that the community are allowed to sit for Birkas Kohanim even though the Mishnah in Megilah 4:3 states that one must have ten people present during Birkas Kohanim, and the Bartenura there writes that this is because it is a Davar sheb'Kedushah. This seems to suggest that according to the Halachah one is not necessarily required to stand for a Davar sheb'Kedushah. According to this, it may be that even for the Sheva Berachos at the Chupah there is not actually a Halachic obligation to stand.

3) I would like to add another attempt to answer this question: Why, according to the Shitah Mekubetzes in the name of the Ge'onim, do we sit for Sheva Berachos? I hope I might be a little nearer the mark this time:

a) There is a related question discussed by the Poskim: Why do we not stand up for the Berachah of Zimun? The Sefer Birkas Chasanim (by Rav Nisan Kaplan shlit'a, Siman 5, p. 34) cites the Chasam Sofer who says that the reason is that the Birkas ha'Zimun is part of the meal and Birkas ha'Mazon. Accordingly, since Sheva Berachos are part of Birkas ha'Mazon, it follows that one may sit for them.

b) A source that Sheva Berachos are considered part of Birkas ha'Mazon is the fact that the chief place where the Rambam cites the laws of Birkas Chasanim is in Hilchos Berachos 2:9, among the other laws of Birkas ha'Mazon, and not in Hilchos Ishus, among the laws of marriage (this proof is cited by Rav Moshe Sternbuch shlit'a, in Teshuvos v'Hanhagos 1:744, DH ul'Rambam).

c) The Chstam Sofer may be found in Teshuvos Chasam Sofer, Orach Chaim #51 (end of DH veha'Rambam). He writes that when a Zimun is made with ten men, it is done sitting down (even though, generally, everything which requires a "congregation" should be done l'Chatchilah while standing), because there the primary Mitzvah is done sitting. (See Piskei Teshuvos 192:22.)

d) Since the Mitzvah of Birkas ha'Mazon is done while sitting, the Zimun is also performed when seated. The same logic applies for Sheva Berachos. Since Sheva Berachos are part of Birkas ha'Mazon, they are said seated.

4) Here is another possible reason for why, even according to the Shitah Mekubetzes in the name of the Ge'onim, that Sheva Berachos is considered Davar sheb'Kedusah, but still according to the Rema (in Shulchan Aruch OC 56:1) -- who says that one should stand for every Davar sheb'Kedushah, one nevertheless sits for Sheva Berachos: This is because the Gemara here (7a) states that we learn that Birkas Chasanim requires 10 people from Bo'az, who took 10 of the city elders and told them, "'Sit down here', and they sat down." This is an explicit verse that for Sheva Berachos one sits.

Kol Tuv,

Dovid Bloom