Didn’t something similar happen with Shania Twain? Her husband left her for her married best friend – and then she got with the best friends husband and married him. These things happen. Regardless, it really is none of your business. They are well within their rights to be seeing each other, they are well within their rights to get married in the same place you did, they are well within their rights to name any potential children with names that you wanted to use for your own – it is NOTHING to do with you. I suggest you and your partner find healthier things to focus on rather than what your ex’s are up to.
Well, you were compatabile (enough) with your ex-wife at one point in time your girlfriend found her ex attractive, and you and your girlfriend are clearly attracted to one another, so is it really that crazy that like would attract like and they too might be attracted to one another?
Maybe they’re doing it to rub it in your faces, but eh, if you’re happy in your girlfriend, what does it matter to you what your exes do?