From THE TELEGRAPH:
Maybe some Christians, but only a few. Certainly not hundreds of thousands. In fact, the only "Christians" who will be "put off" becoming teachers and doctors will be those who are homophobic and those who believe that every word in the Bible is the dictated word of God (a position that is actually untenable as so much of the Bible contradicts itself). To be honest, it would be extremely dangerous to employ a homophobe as a doctor who will be expected to treat gay people and it would be plain daft to employ as a teacher somebody stupid enough to believe that all the Bible is literally true (goodness knows what such a person would teach our children). Therefore, same gender marriage should, as an added bonus, help protect patients and students from prejudice and ignorance. It's a win win situation.