If true, this is an exception. Nikolai I ( not sure, maybe another tsar) had for example an affair with Zinaida Yusupovna ( the elder). Most monarchs (including women) have had endless affairs.
And, Nikolai II did not have a typical monarchic life. He lost power. Same for Louis XVI in France.
Marriage and love were two distinct issues. Consider marriage as a partnership.
And by the way, to come back to our theme, Felix Sr was truly and madly in love with Zinaida Gregoriev(a), until his death in 1928.