Can we delete the word 'sex' from this discussion, because the characters in question were NOT engaging in any sex act at all.
It would be wildly inappropriate for any drama to feature actual sexual content before the watshed, gay or straight.
Furthermore, can we avoid referring to them as 'naked'. They were in bed with the covers drawn up to their chests.
If they had been banging away like a couple of steam engines, then yeah - I would agree with the sentiment that it's not right for early evening entertainment - but it wasn't anything like that.