Clinton was right back in 1993 and I agree with Santorum as well. Sexual orientation doesn't affect how well you can defend the country, but I'm sure there are some soldiers who would be very uncomfortable sharing a barracks with openly gay soldiers. A situation like that doesn't improve the military's ability to do their job, it makes it more difficult.
The audience shouldn't have booed that question. But I think DADT should be reinstated, unless the majority of the United States military officers say there will be no problems if openly gay soldiers are allowed. Gay men should absolutely be allowed to serve their nation if they wish, but I don't see what is accomplished by making sexual orientation an issue.