Maybe we romanticise marriage too much; expecting too much from it? I'm always a bit taken aback by the numbers of women who still dream of big, fairy tale weddings with all of the trimmings..."the best day of their lives" and so on....
Strictly speaking, & certainly traditionally, a marriage was a contractual agreement between two parties, or two families, or two communities ( and in the past a contractual agreement in which the woman was effectively considered property). Marriage was/is more about continuity and stability and the creation of an economic & family unit, than about love and romance - as we now tend to see it, and expect from it.
A love affair is an affair of the heart, not a contractual agreement, and the heart does not follow rules, it follows its own calling. I'm sure most of us have fallen in love and/or had crushes while in a committed relationship with another...whether we act on those feelings is a different matter. Personally, i've always managed to fall in love with unobtainable people/out of limits, or in more recent years, I've developed love affairs with certain cities ( Rome, for example).
A love affair ( not just a sexual fling) makes you feel alive, vibrant.....one sees only the best in the lover...how they match you perfectly...it is irresistible, consuming, maybe even obsessive.....and then it eventually burns itself out.
I was hitching through France once, in my late teens...and I ended up on the mediterranean coast, working on the restoration of a boat that belonged to a couple. A man and a woman. They were both married to other people( and each had children with their spouse) but this was their joint project. they came out to the boat for a weekend every so often and met up with friends of their relationship.
One evening the woman went home to her family, and the man apologised to me for not making a pass at me one evening, after we'd been out to dinner with their friends. He thought I''d be offended if he didn't. What a relief!