I wouldn’t go - straight or gay.  I just don’t get the rationale for doing this.  Also if she had any concerns for your feelings she wouid have called before she posted the invitation.

Glad to hear that you are in a better place.

I absolutely definitely would not go.

No matter how long it’s been or how good you feel, it will affect you. You’ve worked so hard to get here.

If it were me, I definitely wouldn't do it to prove anything, either to her or to yourself.  I wouldn't do it for her (to, say, expiate any guilt she might feel, say, or to provide evidence for the belief that the past's the past and we've all moved on).  I might do it if I was doing it for myself (and not just to prove to myself I could).  But even then, I'd hesitate, because I know by now that there would likely be consequences for me that I couldn't predict ahead of time.  
  I like Abby's point that if you have children together that you are likely to meet the new spouse in a less public setting.

Hi Byron,

I don't think there is any reason to go or to even respond to her.  If there is a response card for attending or not attending, simply send it back with "not attending".  What does going to a wedding have to do with proving to yourself that you're an adult?  I think you have more than proven that.  And as far as being decent and gracious....it seems as if you've already fulfilled that too.  I don't think anywhere in the definition of decent and gracious does it include: and also be sure to attend your ex wife's wedding with her new gay partner!

I think you're being too hard on yourself.  I wouldn't even go to a wedding of a normal ex because that would seem weird to me and also rude to the new spouse.  I don't remember your full background story but unless you two remained good friends and talk all the time and have that sort of relationship (which it doesn't sound like you do if you don't even know the new fiancé) then there is no reason she should have invited you to the wedding.

They just keep finding a way to wiggle themselves back into our lives don't they?  In the end, only you know which decision will make you the happiest.   You don't need to consider her feelings in this decision at all.  It's not about her.  

If you intend to rebuild a friendship with her in the future then you could attend as a politeness.   Do you have kids together?  Does co-parenting require that politeness?

If you don't view her as being part of your life in any way.. I'd toss the invitation in the garbage and forget about it.