Time to compromise. If they rekindle their friendship then don't you think that it would be a good idea that the two of you set up some very fundamental boundaries to this friendship... Like no going on one-on-one outings with each other. Keeping their get togethers when you're also invited. No spending time texting and emailing to the point of distracting away from your union. Maybe you can think of some of your own boundaries that you'd like to see in place that aren't toooo unflexible.
Good luck.