Question: [Yebamoth 61b](http://halakhah.com/yebamoth/yebamoth_61.html#PARTb) discusses the positive commandment of being fruitful and multiplying. As far as I am aware, one makes no blessing before intercourse. Why not?

Please cite sources!

Answer: 
The Maggid Ta'alumos to the Rif on Maseches Berachos, daf 35a (25a in pages of the Rif) says that we do not make the blessing because she may not get pregnant and then the mitzvah was not fulfilled. He also addresses that Onah is a negative commandment and therefore has no blessing.

However, he does indicate that the blessings under the Chuppah do constitute a general birkas hamitzvah.
The Shulchan Aruch ([EH 63:2](http://beta.hebrewbooks.org/tursa.aspx?a=eh_x7690)) actually does quote a beracha from the Bahag, to be made after the first bi'ah with a Betula. The Aruch HaShulchan says that we are not noheg to say this beracha anymore.

However, that beracha is not a beracha on the mitzva, but it appears to be a birkas hashevach. The reason we don't make a beracha on bi'ah is because it is contingent on another party. If she says no, you are not supposed to have bi'ah.