Not currently cheating, but cheated on my wife for a couple of years with a coworker. At first it was light flirting and secret chatting and whatnot. Eventually progressed to making out and then sex in all kinds of places.
Why:
Because I bloomed late. Like, late late. I am better looking at 30 than I have been my entire life and every since I started lifting a few years ago I get crazy amounts of attention. It's like I'm making up for the college years I never had when I soak up attention from women.
Because the woman I cheated with is someone I was extremely compatible with and I was insanely attracted to beyond just the normal 'she's hot', etc.' Ethnically and proportional just rangle every bell I have.
How it's affected my marriage:
It definitely drove down my attraction to my wife. Increased my reliance on porn, etc. But we still had tons of sex anyway because I was horny all the fucking time.
In a weird way it gave me a constant self-awareness of what I was going to lose (my kids, my marriage, my reputation) if things went sidewise. When I cut things off with the other woman I dove back into my marriage with renewed fervor and it's the best it's ever been, sexually and otherwise.
I do love my wife, very much, as weird as that sounds, and I know what I did was wrong. And I have considered telling her. But if I did I'm afraid not just for me but for her; she relies on my income and honestly I don't think she would want to know. I know how self-serving that sounds, but I know my wife better than anyone and I believe it's the truth. If it came out now years after it would certainly be what I deserve but would be disastrous for her life. And thankfully she loves me too.
Save your judgmental PMs, I'm not opening up this account again.