It was Adam and Eve, not Adam and Steve. This is a little "joke" some Christians use to assert the superiority of opposite-sex unions over same-sex ones. But here's the thing: if you really believe the first and only two people on the planet at one point were Adam and Eve, who did their kids marry and have babies with? This, my friends, is incest (happened again if you believe Noah's family members were the only survivors of the great flood).*
A) It did not happen again at the flood, since Noah's family was his wife, his three sons and their three not sisters but wives. Generation after that you get cousins marrying, but that was not incest during the Old Testament (Catholic and Orthodox Church Law makes it incest during the New Testament).
B) It was not incest in the generation after Adam and Eve, because right in that generation siblings were the furthest away relatives you could possibly marry and have babies with.
C) Incest may be emotionally worse or less bad than sodomy. But before and above emotions there is objective goodness and badness of an act. We Catholics really believe the "having babies with" part is essential to the goodness of the marital act. We believe a married couple using condoms to avoid having children while having sex is committing a worse act than an incestuous couple making a baby. Adam and Eve really could make babies with each other. Adam and Steve cannot. Adah and Eve cannot. But Adam's sons could with Eve's daughters.
Mouffetard Library, Paris
St John Maria Vianney's Feast Day
*Ten More Cliches Christians Should Avoid, nr 8.