Someone very close to me went through this a few years ago. She never saw it coming and was COMPLETELY blind-sided as you were.
If that's the kind of man(?) he is, he isn't worthy of you anyway. You can do WAY better, and sometimes learning to love ourselves is a step up from being with a husband or wife who would cheat and/or leave us for another (and you can take comfort in knowing that when he gets bored with her, he'll do the same thing to her, I can almost guarantee it).
To the person who found her husband in bed with another man: I am so sorry to hear that, too. When I was 21, I dated a guy for a while, and we had the greatest chemistry, kissed a LOT, were very affectionate with one another, but then I found out about Sam... and Eric... and Alex... and this whooooooole other "life" he had that nobody knew about. Luckily, we were only together about 4 months, but it still hurt like the dickens.
I am now quite happily married to a man who went through something similar with his ex-wife. His ex-wife left him for another woman, apparently had been a lesbian her whole life but never admitted it to herself and tried to force herself to fit into the husband-and-kids-and-whole-straight-life thing & it just didn't work for her. She's good to the kids, though, and she and my husband still get along well. They're friends now, but it took a lot of pain for them to get to this spot.