To give someone the benefit of the doubt is to default to the belief that their intentions are honest, and not assume malice when there is uncertainty or doubt surrounding the circumstances.
John didn't pay me back the $20 he owes me, but I'll give him the benefit of the doubt that he simply forgot and isn't trying to rip me off.
Two friends who trust each other enough to engage in sexual activity without fear of hurting the other's feelings. Ideal scenario for folk who are not interested in a serious relationship, or who do not have time for one. Not a boyfriend or girlfriend; neither party has to refrain from dating other people. Also not a word tool for a player to have sex with women he does not care about. A smart alternative to random hook-ups.
John and I are friends with benefits. We just hang out, and have sex. It's easy, now that we've established the ground rules. If we stopped having sex, we'd still be friends.
Student A: Did you hear about Jake and Alyss?
Student B: No, did they finally fight?
Student A: No! They had sex!
Student B: Enemies with Benefits. Sweet.