As I understand it, most marital acts don't result in conception. Lately I've been wondering how the fact that procreation is the natural end of marital relations fits into the qualification that events that happen by nature occur "always or for the most part" (Physics II.8.199b24). I just ran across this old, but very interesting post of Jimmy Akin ("Higamus, Hogamus," June 13, 2004) that may have at least part of the answer:
It turns out that as a result of the marital act, genetic material from the husband is permanently absorbed by the wife's body and becomes part of her--a dimension of the "one flesh" union between husband and wife that previous generations have been unaware of.
One of the ways these problems are reduced is that having absorbed sufficient quantities of the husband's genetic material better enables the mother to perform the immune modulation needed to allow her child--with its foreign genetic code--to exist in her body without her immune system trying to eliminate it.
The BBC article Akin links ("Sex 'primes woman for sperm'," 6 Feb 2002) contains this thought-provoking sentence: "The theory could partly explain why humans have sex even when they aren't trying for a baby."