I was looking at vol I chap. 19 in the book of Rav Nahman and noticed a few things. One is the approach that he takes towards sex. The general desire for sex he calls ''desire for adultery''. He is not thinking at all the sex in marriage all of a sudden becomes holy. It might be permitted but still has that aspect of ''desire for adultery''.  But this is not new to me. You can see in the Mesilat Yesharim that sex can be holy once one has gone through all those preliminary steps enumerated in the braita of R .Oinchas ben Yair. Mainly however I would say that a lot depends on intension.

In fact it makes sense to learn the sidur of Rav Yaakov Emden for newly married couples which goes through this subject in detail.

besides that i have no idea why people think marriage makes anything better or holy. i have repeated plenty of times the story about the friend of Joshua [who conquered  the land of Canaan] Caleb ben yefuna in Chronicles chap 2 verses from around 43 to about 50.  it is hard to tell how many wives and how many girl friends he had but clearly they were quite a bunch. and hewas not some low life. וימלא אחרי שם ''he went totally after God'', [see rambam at the start of laws of marriage and the raavad and ramban there. ]