A few months ago, I was revising Entwined and trying to work through a major plot point: the fact that my hero initially has a girlfriend and baby but leaves them when he goes to do research a few states away. The relationship is a strained and unhappy one, but it's still a relationship. While away from home, he falls for the heroine. I wanted some feedback on how palatable this hero would be to readers, so I posted my dilemma on a writers' forum I frequent.
The response was interesting, to say the least, and still draws comments every few days. Far and away, most people said they'd be turned off by such a guy. Romance readers don't want a hero who cheats. Beyond that, the traditional romance genre doesn't allow for a hero to be a cheater; the story would be unpublishable. Of course, I don't write traditional romance for a traditional publisher, but it's still something to keep in mind.
Others said that if it was more along the lines of women's fiction, they'd be more open to characters who don't follow the typical/predictable mold. They still wanted a clear break between the hero and his girlfriend, though. Quite a few said that because so many women have been cheated on in real life, they wouldn't want to read about a guy who did it in a novel. There could be allowances for emotional growth, but the character would have to be well explained for readers to buy into his change of heart.
A few other commenters (definitely in the minority) said they wouldn't mind/judge a hero like that automatically because relationships ARE complicated, and because it isn't realistic to think that infidelity doesn't occur in matters of the heart.
Overall, it was interesting to see the range of responses, but it did reinforce an important notion: KNOW WHO YOU ARE WRITING FOR. Know the rules and expectations of your genre, know if you can break them or how far you can bend them, and keep your reader in mind.
Always good points to remember. Happy writing!