Re: What to do?
I totally agree with TDSC60. Tell her you know that she's been trying to meet up with guys on these girls nights out and that she either stops them IMMEDIATELY or she moves out of your house. See a lawyer to find out your options for divorce and don't make a secret of it. Start the ball rolling. The only reason not to do this is if you think she might have actually met up with someone - in that case, start snooping NOW.
If she steps up to the plate and actually complies (very unlikely) then you will need to watch her very closely for a LONG time. Chances are she will call your bluff though. Be prepared to follow through with D. Of course that isn't ideal, but staying with her 'for the sake of the kids' will do nothing but make you (and the kids) miserable. Not to mention you're accepting being a cuckold. And giving the kids a really bad example of what marriage should entail.
Oh, and I recommend STD testing and no more sex until you KNOW what she's been up to.