I, personally, think one night stands are irresponsible and stupid about 98% of the time. Mainly because teens account for the majority of the people who do this and that is always dumb, I don't care how careful they try to be.
If you do not know someone well enough to know how they feel about pregnancy as a possibility or if they have diseases or not, then you've no business sleeping with that person. At all. I say this because men who do it are the first to whine when the chick wants an abortion or he doesn't want to pay child support and girls actually are surprised when the guy doesn't stay interested in them because we have wildly different ideas than guys do, but won't accept it.
The only time the above is not really an issue is when the two parties are adults-and I mean like 30 . And even then, it's probably not a very good idea.
All in all, I don't think they are moral or immoral, I just think it shows a lack in character and maturity.