You can be cordial with somebody you have slept with but I don't think you can actually be friends. As far as communicating with them or spending any time, esp if feelings were involved. I've tried in the past and they always end up wanting to smash again. And if i was single I usually did it. Once you cross that line of being intimate , you cant just be "friends". Also if you in a relationship, I don't know many partners that would be ok with their SO being friends with somebody they was fucking.