When lovers are good friends first and foremost is when sex can be a lot of fun. Then what a couple does is what they've learned they both like. When there is no issue about respect (i.e., when it's firmly there), then couples may start to treat sex as an opportunity to play.
Now, if one realizes consentual sex between adults isn't shameful in any, way shape or form, then one is free to play with one's lover in great many ways. Some of the varieties of sexual play, especially that which can develop over time as a relationship matures, might appear obscene to someone relatively naive about the play aspect, or who have self-esteem or guilt issues around sex.
But what might appear obscene from the outside is really due to the fact that some people like raunchy sexual "play." A woman, for instance, may "play" a whore in the bedroom with her friend-lover, and feel assured that outside the bedroom he knows they were just playing, and that her (and his) wicked behavior during sex has absolutely nothing to do with her (or his) character. Because the friendship is there, only issue between them is discovering the ways they both enjoy, or at least feel comfortable, playing.
One of the best things I've ever learned for enjoying the experience -- beyond respect, friendship, openness, and non-judgementalness about sex -- is how much fun it can be to figure out what turns on my lover, and make her hot as a firecracker doing that.

There's more to be had through that vicarious route than most (men especially) realize. I suppose that's why I like to cook for people too.
