Lose his job? No. This isn't male comradery speaking, just an honest opinion.
As others have said, there is a huge double standard in nursing. As a male myself, I am very reserved and cautious in discussion. Even if I feel I know the person well enough and the boundaries, I know the implications of sexual harrassment accusations and the weight that follows. I have not personally been accused, but I know some that have (some with and others without merit).
Here is my impression: you should have addressed it with him immediately on the spot if it makes you uncomfortable. You don't have to come off as disciplinarian, just a simple "sorry, that really isn't appropriate" will suffice. Sexual harrassment is just that - harrassment. As in repeating the act with full knowledge that it makes you uncomfortable. He certainly overstepped his boundaries, but you need to declare those boundaries clearly. Unprofessional? Yeah, as are most non-work conversations at the nurses station. You are justified in writing him up, although it would have been more effective to have just spoken directly to him.