"Friends with benefits" is not the same as "no strings", and I think it is immoral to interpret it this way, particularly if this is a real friendship and the friendship comes first. "Fun" has to be tempered with serious responsibility to both yourself and your friend, as discussed below:
I agree with much of what Epicurus said about friendship. Friendship is a social contract, agreed to by both parties, based on a mutual commitment to avoid both suffering harm and inflicting harm - including both physical and emotional/psychological harm.
In modern terms, this translates into both parties maintaining healthy personal boundaries.
To avoid suffering harm means to be assertive, non-aggressively assertive, and not allow a friend to cross reasonable lines, which would constitute abuse - and to make continuation of the friendship conditional on the other party not becoming abusive. If they become abusive, ditch the friendship, and it may also be a good idea to maintain no contact.
To avoid inflicting harm means to take one's friend's emotions seriously, and that includes (as an example):
- Finding out how they really feel through pro-active enquiry.
- Empathising with them (attempting to place oneself in their shoes).
- Communicating healthy and reasonable boundaries, directly, not assuming they should be obvious.
- Making a commitment not to transgress those boundaries (including in anger or hurt). This also rules out a tit for tat if one feels hurt - communicating the hurt openly has to replace retaliating with spite.
- Keeping this a mutual arrangement, on both sides, keeping the relationship balanced.
- Keep the relationship and power dynamic "horizontal" and egalitarian.
- Do not become abusive.
- Modern research suggests avoiding like the plague the "Four Horsemen": Criticism, Contempt, Defensiveness, and Stonewalling. These are so toxic they can very accurately predict the end of a friendship or relationship.
- And so on...
Now, the exact "rules" one may decide upon may differ from person to person, but in every case they ultimately boil down to the same thing: A mutually agreed-upon social contract, put in place to guard against both suffering harm and inflicting harm. "Harm" in this case also includes emotional and psychological harm (which is ultimately in the body too), including serious hurt.
"Neither harm nor be harmed".
Now, THIS is the emotional condom I had in mind for FWB (and e-FWB) relationships. A Neo-Epicurean flavoured condom.