A wise old friend told do not have sex w/ anyone you do not have intentions of marrying. Accidents do happen, even in the safest of places on pratices.

If the girl you are thinking of is a new relationship you should not be thinking of sex w/ someone you don't know all that well.

As for the guy, if sex changed your friendship then you have to be friends before you can be lovers. If the friend thing was weird after sex it will be more weird after more sex.

Thats just my opinion. Best of luck w/ what ever you decide.