Bishops writing with clarity and concision:
It is not discrimination if a person who cannot swim is rejected for a position as a lifeguard or swim instructor. It is not discrimination when a man who cannot lift 25 pounds is not hired as a piano mover. And it is not discrimination when a man is not permitted to play in a women’s tennis tournament. In the same way, noting that two men or two women cannot be the procreative, comprehensive union that marriage is, is not (unjust) discrimination.
Only a man and a woman are capable of sexual activity that may yield children. The government has a strong interest in protecting the right of those children to a mother and a father and in reducing the likelihood that those children will become wards of the state. The civil law of marriage (until recently) served both these interests by legally bonding adult couples to any children they may create, and to each other.
On the other hand, the sexual activity of two persons of the same sex never yields children, so the government does not have a very compelling interest in getting involved. The government does not care who your best friend is; you don’t need a license for friendship or cohabitation. It would be eminently reasonable, and in no way unjust, for law to distinguish between same-sex and opposite-sex relationships.
Likewise, it is reasonable that a professional serving a customer can distinguish between activities that express approval for same-sex sexual behavior and those that do not.
(source: FAQ Series: Isn’t that Discrimination?)
