I don't know Oregon law, but that would be the case under common law, and remains the case in many if not most states. He would also be the legal father if she'd gone out and gotten pregnant from a one night stand and DNA tests proved conclusively that he was not the biological father.
The origin of this obviously predates DNA testing, though it was applied even in cases where the racial characteristics of the baby obviously ruled out the husband as the biological father. Remember that under the original concept of marriage in English-U.S. law, the wife was not a full-fledged person, since she was female and not eligible to enter into contracts on her own behalf (among other things). The husband was the only legally responsible person in a "nuclear" family (the children being below the age of majority), and he was therefore legally responsible for the wife and children -- that included being responsible for the wife getting pregnant by another man. The law was also not willing to give any man other than the husband any rights whatsoever to a baby which emerged from the husband's wife/property, because that would have been interfering with the husband's ownership rights to his wife.