Civil unions provide the best of both worlds
The best legislative outcome is always some form of compromise between the two sides that gives everyone what they want.
Civil unions can give everyone what they want. Same-sex couples can enjoy the legal and expressive components of marriage. Meanwhile, those that oppose gay marriage on religious grounds or over child-rearing concerns are able to maintain the sanctity and institution of marriage.
Any discrepancy between the legal rights of a same-sex couple and a heterosexual couple surmounts to discrimination. Therefore, civil unions are still a form of discrimination as they do not grant the same legal rights of marriage to a gay couple as a heterosexual couple. Civil unions also don’t bring the same non-governmental benefits. The private sector often treats those in civil unions as being single, preventing access to lower insurance premiums, healthcare benefits employment opportunities. While civil unions offer an attempt to provide homosexual relationships with marital rights, they can cause as many problems as they solve. Creating a new marital status (single, married, or in a civil partnership) will instantly flag a gay individual to prospective employers, government authorities or medical practitioners. Not only can this cause some unwanted outings, it opens the door for discrimination. The only way to avoid this is by giving same-sex couples the ability to marry.
[P1] The preferred legislative outcome should be the one that addresses the concerns of all sides. [P2] Civil unions address the core concerns of all sides. [P3] Therefore, civil unions should be legal.
Rejecting the premises
[Rejecting P2] Civil unions don't give all sides what they want.