Is it unsafe to have sex with an uncircumcised person?
It is just as unsafe as having sex with a circumcised man; you can become pregnant to both, and you can also catch STI's from both, but you shouldn't have any problems if he wears a condom.
It just means the guy has a little more skin on him is all. Not different in any way.
It's the same as doing it with a circumcised guy.