Why you shouldn't sleep with a friend
Sex and/or relationships with a guy/girl you just met at a bar, and sexual relationships with a good friend, are two different things.
WanjiruWarobi 
It could ruin your friendship
The friendship dynamics can change. What was once a very mutual like and respect for one another can get very weird very quickly – especially if one of you had feelings for the other.
The other matter could be that he/she is now expecting you to act a different way toward them, whether it happens that they don’t want to hear about you seeing someone else, or they get jealous in general.
When they get someone else
You sleep with that person and then they start dating someone else.
It becomes awkward from here on when they introduce you to the new guy/or girl as their close friend but you both know you are not just a good friend. There's also the fear of that partner finding out later that he/she isn't just your friend.
You are therefore faced with confessing and losing one of your friends or partner, or facing your partner when he/she confronts you with the truth that he/she found out from someone else.
You can't give unbiased advice
It becomes strange to go to this person that you had sex with, to talk about your heartbreaks and dating life. Maybe it used to be normal to seek their opinions before the sex happened, but not anymore.
Their opinion somehow appears clouded by their personal interests in you.
