There is a mitzvah to populate the world, pru urvu. For most mitzvot, there is a bracha we say before we do it. So why do we not say a bracha before sex? Also, whenever one benefits from anything, he says a bracha for it. So why is there no bracha for sex?
The Minchas Chinuch Mitzva 1, number 14 proves from the fact that if ones children pass away without leaving children then the parent has the mitzvah of pru uruvu reinstated, that the mitzvah is not the act of sex creating the children, but rather the mitzvah is to have children in existence. (With this idea he explains why a convert who has children from before his conversion does not have the mitzvah even though they were born when he wasn't a bar chiyuva). He further explains in number 30 why kavanah is not required during sex since it's only a hechsher mitzvah (like buying an esrog) and not the actual mitzvah. (He concedes that Tosfos seems to argue with this and he says he doesn't understand)
Possibly this would explain why no blessing is required as it's only a hechsher mitzvah.