Jake If he is casually asking for sex, its because it already happened. Plain and simple. Sorry bud its not cool, but at least I think you now know enough to STOP snooping, and start getting control of you, your emotions, and maybe even the situation. I'll ask again have you read no more mr nice guy, or are you still praying this thing will spontaneously blow over?
Stop the snooping now, I think deep down you know your answer. Now its time to stand up for yourself and for your marriage. She clearly does not respect you anymore Jake, otherwise she wouldn't be talking to OM about sex, or at the very least she would make it clear to him that "it's not like that"
That's funny she'll tell you till her face is blue that OM is just a friend but why wont he say that when OM asks for sex.
Jake your wife is in serious dream land and she needs a heavy dose of reality. Some of that reality should include that she cannot treat her husband like dirt, and expect him to stick around. She treats you like this because she takes you for granted, she'll only pick you if things don't pan out with OM. In her eyes you are not first round pick material.
So here's my practical advice, read [edited by dbmod, reference not recommended nor allowed], think of what is keeping you with her, reevaluate that in the face of your new discovery. Accept that thing are as bad as you feared, then figure out how much you will tolerate.
If you stand by and do nothing you are just encouraging to be a serial cheater. Find what YOUR boundaries are, no staying overnight with the opposite sex, no friends of the opposite sex that are exclusively hers, etc., etc. Yes she'll threatened to leave, yes she'll scream and maybe run away to OM, what's the worse that could happen, she runs to have sex with him? Wait she's already doing that.
Jake I think you seriously need to consider your limits.