Why would you willingly hurt someone you love? Which raises the question, did they ever really love you in the first place?
Speaking for my boyfriend here, I can't explain why he chose to do what he did. I understand the reasons why he did--they stem back to childhood abuse and neglect, a need for attention and validation, and a stunningly strong ability to bury his head in the sand and hide from problems that has been a recurring theme throughout his life. He absolutely had constructed an alternate life in his head, or at least part of one.
Those things don't make it ok in any way, shape, or form. But I know he loves me. I knew it both times he chose to cheat on me, and I know it now, three years later after the last time I confronted him. We've both been to therapy together, and he's been seeing a therapist on his own for the past year, and I've seen him make huge strides with some of his issues. Because he's been willing to make that commitment, that goes a LONG way toward rebuilding my trust in him.
It helps that I'm not hung up on monogamy. I understand that one person can't always meet all of the other's needs. The #1 thing I was upset about was the lying and deception. We've worked hard to build that trust with each other (because I'd broken his trust through some of my behaviors--not cheating, but with other issues I'd had) and I really do think it's paid off.
These days, we're pretty stable and happy. We take everything a day at a time, and work on planning a future together. We want to be together in 5, 10, 20, 50 years. We can't promise that we WILL be. But every day we get up and act toward each other in a way that will make that happen.
If he cheats again... I don't know. I really don't. Again, for me it's not about sex or desire, it's about honesty, and his ability to tell me what's going on in his head and what it is he needs. I guess I'll cross that bridge if/when I get to it.
In the meantime, I'm happy where we are now and where we're working toward together. We've both come a long way, and he's worth it. I know it may not sound like it, but he is.