User blog comment:Miskos3/Fairy Tail Chapter 508: Pleasure and Pain, Review/@comment-27547436-20161031171812/@comment-7418318-20161101132658

There are ways to imply it without showing. For example have them kiss, then fate to black and then show them waking up the next morning without clothes on. That's still well within shounen boundaries and yet also makes it pretty clear what happened between panels.

Of course in Zeref and Mavis case that wasn't really a choice due to the curse. If a death wave isn't triggered at the first kiss or at some point during their "embraces", it's hard to justify one going off on the morning after.