Same-sex couples should not marry if they do not love each other. They should not marry solely for money. In short, the reasons for not marrying are the same for same-sex couples as for any other couple.
In some jurisdictions, they can legally marry the same as any other couple. In other jurisdictions, it is not legally possible for them to marry.
As an expression of their love and commitment to each other.
It may not be legal in that jurisdiction.
Same sex couples can't marry in some churches because the leaders of those churches are mentally challenged homophobes and they think it is against nature. People are stupid sometimes.
In some jurisdictions, same-sex couples can legally marry. In the places where they cannot, the reason is that either the local laws explicitly prohibit it or the authorities who certify marriages interpret the law and tradition to limit same-sex marriage to opposite-sex couples only.
LoveCompanionshipDesire to raise childrenFinancial interdependenceThe reasons are the same as for opposite-sex couples.
Same-sex couples have a constitutional right to marry in the United States, Canada, Mexico and Brazil. In some other countries, they have a legislated right to marry.
The right to marry and form families is considered a basic civil right. In some cultures, however, governments claim there is are special reasons why same-sex couples should be excluded from this particular civil right.
under 1% marry their college sweethearts. but it is very possible and some college couples do actually commit to the marriage for a very long time
There is no legitimate legal argument for limiting marriage to opposite-sex couples only. Denying same-sex couples the right to marry is a violation of the United States Constitution.
Some counties are holding off on allowing same sex xouples to marry till they are instructed to by the state, but many are marrying same sex couples as we speak!