Civil Unions don't give the same rights as marriage
First off, civil unions are not recognized by all states. So if domestic partners leave the state, of which they got the union, and crossed the boarder into another state that doesn't acknowledge their union they union would becomes invalid. So say they were to go on vacation to Florida, and one of them were to get injured to the point where he had to go to the hospital, his partner would not have hospital visitation rights. Also, if they only had insurance through the uninjured partner's side, the hospital would not accept it, because their union does not exist. A callous person might say after hearing this, "Well, just don't go to those state." 31 states do not recognize civil unions. It is wrong to say that domestic partners cannot enter 31 of the 50 states, because of the fear of losing their spousal rights.
Secondly, even in the states where there are civil unions, the Government Accountability Office states that there are 1,138 different rights/benefits that married couples have over domestic partners (Government Accountability Office ). I believe that is called discrimination -- the unjust treatment of a different category of people on the grounds of race, age, sex, or sexuality.