I think the question you have to ask yourself is "Can my wife fulfill my marriage needs both now and into the future?" Now that you know she is a lesbian, and that she is physically attracted to women in a way that she can never can be with you, is that enough for you? Is it enough for you that she came back for the time being? Do you trust her? Can you trust her that if she is at a roller derby she isn't looking longingly at the women there? Are you setting the best example to your kids of what a marriage relationship should look like?

For me, that last one was the kicker that did me in. I don't want my kids to look at my marriage as the example of what they should strive for in their lives. I don't want my kids to settle. I want them to have a loving partner who reciprocates their love in the same way and the same amount as they love them. If I settle and lower the bar, am I telling my kids that they somehow don't deserve to be happy?

Thanks for the replies.  I've never thought she was 100% lesbian, but I guess none of us ever did?  But even while she was falling in love with this other woman we'd still have sex and I would still make her orgasm.  I don't know much about it, but I feel like if she was 100% lesbian she wouldn't be able to do that? (I don't think I could do that with another man, no matter what). 

I went with her to couples-counseling last week and she told her therapist (and me) that she made the biggest mistakes in her life, and all she wants is to have me back.  She said she realized she can't live without me.  She even parroted back to me a lot of the same arguments that I made to her when trying to convince her to stay (I'm not proud of it, but I was literally begging her to stay before I finally was convinced and let her go).

I think the main issue I'm having is I don't want to take her back and resent her forever.  If I take her back I want it to be on 100% equal footing, and not having her feel like she "owes me" or anything.  I'm not sure I can get to that point with her again because of this.  I'll always wonder which of her lesbian roller-derby friends she'll leave me for next?
 

 
  There's a term for women who are willing to do what you are contemplating: doormat.  And if you act like a doormat, you'll get treated like one-even if you're a man.  Look at the sequence, and what she did, and what you did: She told you she liked girls, you told her to explore.  She did; she fell in love with a woman.  She told you she was setting up dates with her lover; you told her not to; she did it anyway.  She wanted a divorce; you didn't, but you gave in.  Then when her hot and heavy affair had run its course, you asked her to your brother's wedding as your partner, and now, she wants back in.  
    You're not a person she loves and respects, you're her soft place to land, her fall back position, her consolation prize, her "I"ll settle for that for the time being" guy.  What she really wants is not you, it's to suffer no consequences for the choices she made, and she's betting she can get you to let her back in.  Because the trust is gone, and you're waiting for the other shoe to drop (another affair), life in that marriage is never going to be comfortable for you. 

If you can swing it, I would recommend individual counseling for you both before making any big decisions. Your needs are important, too! There may be some free or low-cost resources available for your wife (especially if you're in/near a city). I agree with others that trust will always be an issue — and, in the end, you'll have to decide if you're willing to live your one precious life waiting for the other shoe to drop.
Best of luck to you in the days ahead. Post here any time you need an empathetic ear. We've all lived through some version of this — and truly feel your pain.
xo j