The 5 love languages book was a good read on this subject. Those feelings of physical attraction in the beginning of relationships is chemical and designed by nature for mating instincts. I've read in several places it can be short or at the longest last for 2 years. Real love comes after that and different people have different ways of feeling loved (the book goes into 5 ways) but a lot has to do with how you felt loved as a child.
Anyway, my husband's emotional affair eventually led to her kissing him and he said at that point all reasoning went out the window. This went on for a few weeks. I didn't know but all the signs were there and broke down one night. He ended it the next day (and left his cell phone home by mistake) and I found out that same day by looking through his phone.
When he talked about the excitement of it, my response was that's called dating...and I guess perpetual daters that don't want commitment just want the high in the beginning. If that's what he wanted, he definitely should not be married to me because I signed up for the monogamy deal. He said he didn't want that, but while he knew it was wrong, could not stop himself once it started and if I hadn't broken down it would have led to a full sexual affair... long pause while I pulled my heart out of the Cuisinart and shoved it back into my chest
He probably spent the next week in a state of confusion and did some soul searching and felt that part of the attraction was that he felt sorry for her...being divorced twice, latest ex was an alcoholic and just lost his job, cable got shut off, etc. and she just went on about how she needed to meet someone like him. I, on the other hand, started working at age 12 and never relied on anyone financially (dad has extreme gambling problem so never had that luxury of consistent electricity, heat, clothing, etc.) so he didn't feel that I really needed him. When I broke down, he realized how untrue that was and started to resent her thinking she was just looking for security and not really being a friend to him by encouraging him to leave me and stray (although he does take full responsibility for his role in it)
At the end of the day, he felt I didn't really care about him or what he did and I thought my kids (all from a prior marriage) were just too much for him and he'd eventually leave, so the wall got bigger and so did the distance between us.
We decided right away we both loved each other and wanted our marriage to work, and since have had many good talks and have both made sure the other feels loved. We started dating again and working out together and love being with each other. I am actually more attracted to him than when we first met because we both have opened up to each other in a way we never did before (we are both fairly shy, private people) and in a very long-winded roundabout way, in my case sexual attraction can be rekindled by feeling reconnected and loved by your partner.