Dropped reading this like... sixty chapters ago when the guy absolutely refused to fuck his harem with anything besides tentacles.
Thing is, I understood his resistance to it, at first: he's not ready to be a father and, initially, they didn't exactly have the greatest financial security, so having a kid would be irresponsible and, well, babies are the natural outcome of unprotected sex (glossing over the fact that anal is an option, but most authors (Japanese or otherwise) seem to ignore its existence).
But the joke got dragged on
waaaaay too long; he mentions wanting to hold off until he can get condoms
once and then never brings it up again, figuring maybe one of the girls in his harem might know a spell similar (hell, with enough imagination, he could have even turned his tentacle spell
into a condom spell).
Really, as usual, you can blame it on the editors and publishers; it was just the editor's way of saying "we're okay with all but explicitly stated tentacle rape in a shounen series, but gods forbid we show two consenting people having sex" and the author figuring they might as well roll with it.