Most Helpful Guys
It is a combination of security and thrill. We all want security and people to share our lives with, but we also love being adventurous and seeking thrills.
In a secure relationship you have someone who will hopefully listen to you, support you and be your equal in every way a relationship should be with stability and security with a roof over your head and dependency. What this does is promote routine and long haul which becomes too boring for others. People want to break routine to experience change and fun.
Thrills and adventure with a side piece keeps things fun and interesting and offers an escape from routine a secure and easy table relationship brings. At the end of the day for people who see this it is no strings and commitment, although it could be if they go through so much work to keep it secret. Anyway, it offers the escape like I mentioned with no commitment, but when it's done they can then go home to their ideal partner with no belief of consequences and have that secure lifestyle that they otherwise wouldn't have if they were only single.
It's messed up and perverse, but really a shame since you would imagine the in the perfect world that the stable relationship can communicate enough to keep things fun to prevent that from happening, but that's just the way I see it.
When you stay single and fuck around, you get the sex and that thrill, but you don't get the perks and the emotional benefits of being in a relationship. Cheaters want to have both. They know cheating is wrong, hence the retarded excuses, but they still do it because they want to have the cake and eat it too.
Most Helpful Girl
A lot of the so called idea is they like the routine with their current partner, just not the sex. They like having dinner cooked or whatever.