What does this have to do with, "... people try[ing] to control how others feel and who they love ..."? Marriage is a privilege, not a right. The state gives this privilege to suit its interests, not to make people happy. The interest of the state, in this case, is to promote a healthy and stable society; which means a healthy next generation. The best case to achieve this goal is to promote a stable male/female couple who will likely a) have children (no other combination can), and b) hopefully raise this child to be a good part of society. Hence, marriage being between a male and female. (Note: there is NOTHING religious about the above at all. Other arguments could be made if we want to go down that road, such as religious marriage in church, temple, etc. I'm strictly talking about secular marriage here.)
No one is prohibiting anything. If people of the same sex want to form a committed relationship, no one is stopping that. In many places, they can even get packages of benefits/'rights' which give them solutions to such issues as estate planning, hospital visitation, and partner benefits at the work place.
Also, the argument is not that a single male or female, or male/male, or female/female couple can't raise a child with good results. And, certainly some of these arrangements are much better than bad male/female couples. The argument is that since the male/female couple is the best (numerous studies indicate this), this should be what the state promotes. We don't want to promote sub-optimal or make it equal to in how it is viewed, as this encourages the sub-optimal (and not just that people will then take advantage of the opportunity, but also in how society views it).
(Again, note: we're not talking about value of the people or relationship, or morals, etc. here... simply how it is viewed as a benefit to the state to achieve their goals, which is why they would give such a privilege in the first place. The state doesn't really care about - or at least probably can't afford to - give such a benefits package just to make Adam and Steve feel good about their relationship.)