The phrases "on my behalf" and "in my behalf" have become interchangeable and I bet few people know the difference. "In my behalf" means basically working as a representative. The military is working in behalf of the president, our commander and chief. "On my behalf" does mean that he will benefit from the military. Maybe his mistake will give everyone an English lesson and make my 7th grade English teacher proud (she drilled the above into us).
As far as Obama, he knows that he can not win the argument with the the economy. So I am looking at this as a campaign speech. He is trying to make a stand that he has been wish-washy hoping to persuade voters to forget about the economy. He is also throwing in the fact that he is in charge and under him we got Osama and the ending of the war in Iraq. I think that is going to come out as his biggest accomplishment.
As far as marriage, it is a religious institution. However, the government has already adopted policies for married people: you go to the government to get a marriage license, you pay taxes as a married person, and you get certain legal protections and benefits from being married to name a few. So to deny two consenting adults from getting married and expressing their religious rite, you are taking away peoples rights. A gay couple wants to have the same rights as a heterosexual couple in "marriage" where the term marriage is a government term that your religious beliefs may not allow certain couples to be married.
It is what you learn, after you know it all, that counts.