It is a Constitutionally-protected right to have religious freedom. To force a private religious-based organization to either choose to go against their religious beliefs or to stop finding good homes for children in need is to force them to forego their Constitutionally-protected freedom of religion. There are plenty of other non-religious-based adoption organizations who allow those who are homosexuals and lesbians to adopt.
Some adoption agencies require a minimum age for an adoptive applicant(s). Some require marriage. Some require a marriage of at least 5 years. Others have financial income requirements. Every agency is doing what they feel is best to protect the children they are placing. If it is a private agency (not government ran or funded), then they have that right according to our founding fathers.
If the government gets to dictate the way citizens run their private businesses, then we are living in a socialistic society, not a democratic one.