Wrong.
A violation of the 14th Amendment occurs when persons of a given class are denied the right to due process and equal protection of law predicated solely on who they are, absent a rational basis, objective, documented evidence in support, and a legitimate legislative purpose.
Consequently, laws denying same-sex couples access to marriage contract law for no other reason than being gay violate the 14th Amendment.
Laws prohibiting incest are perfectly Constitutional because they are rationally based, are supported by objective, documented evidence, and pursue a proper legislative end:
“Incest laws aim to promote security and unity with the family, and to prevent the genetic problems that often occur in babies whose parents are related.”
Incest Laws and Criminal Charges | Nolo.com
Moreover, laws prohibiting incest do not violate the 14th Amendment because they are applied to everyone equally, seeking not to disadvantage a given class of persons through force of law.