A controversial, anti-gay billboard on a Virginia interstate was defended by its advertiser last week.

The sign states that "Nobody is Born Gay," and cites a twin study finding that two people with the same genes can have different sexual orientations.

The Parents and Friends of Ex-Gays and Gays (PFOX) commissioned the billboard to promote their message of gay conversion. LGBT advocates were angered by the sign.

"I am shocked and really disappointed that at the end of 2014, we have a billboard in the middle of our city that says that kind of hate," LGBT activist and Rosmy Executive Director Beth Panilaitis said.

"No young person or adult should be walking down the road and see something that says that they are not okay as they are."

Richmond Gay Community spokesman Bill Harrison said the billboard suggests that there is something wrong with the gay twin.

"We do not know why people are heterosexual," he asserted. "Maybe when we figure that out, we can determine why some people are not. What we do know is that this type of therapy has caused many people to commit suicide."

PFOX Executive Director Regina Griggs said that the billboard's intent is to inform, not criticise.

"We find it interesting that the attacks against the billboard and ex-gay community have nothing to do with the facts," she said in a statement.

"Identical twins have the same genes or DNA. They are nurtured in equal prenatal conditions. If homosexuality is caused by genetics or prenatal conditions and one twin is gay, the co-twin should also be gay...Because identical twins are always genetically identical, homosexuality cannot be genetically dictated. No one is born gay.

"Every person seeking positive life change needs the love and support of family, friends, the community and the church. Thousands of individuals have made the decision to leave the homosexual life and will attest to the fact that change is possible.

"PFOX believes people deserve to know the truth and believes respecting the lives of those who have made a decision to seek change is part of building a tolerant society."