My husband had an emotional affair with a sub-contractor. He started spending longer hours at work, had mentionitis - she was apparently the best coder he'd ever met, but completely unable to see it in herself.
He'd get home, shovel his dinner down (dinner which had been in the oven for hours and dried out), barely speak to me or the DCs, then open his laptop and 'work'.
We own our own business - equal partners now, but 18 months ago (when it started) he had 100% shareholding, although I did the admin and the books. July last year he told me he no longer needed me, which at the time was a relief - he was hideous to work with. I should have twigged then that there was something going on.
He started staying up later and later as she was based in Canada - so totally an EA, until he decided on a whim that he 'needed' to see her 'or else she would go and work for another company'. So, at the drop of a hat, 2 weeks before Christmas, he went. I could do nothing to stop him. The company lost £5k over those 4 days - flights, hotels, expenses. His time. He was completely taken in by it.
I believe he went to make the affair physical. But I also believe him when he said he couldn't go through with it.
What hurt the most were his comments when I finally found out. They were 'just friends', he'd done nothing wrong, and during what I thought was the unravelling of our marriage, she was 'everything' to him - they had a 'deep emotional connection'...one that he'd apparently never had before. (These, by the way, were things he told me at the beginning of our relationship).
Sorry for the long ramble, but I firmly believe that it would have been much easier to leave him had he had a PA. This EA shit made the decision making much harder - as @saltandvinegarcrisps1 says they are foreplay. They are like the beginning of any romantic relationship - remember how it was when you first got together? Flirty texts/emails/notes/phone calls. Seeing one another, but not yet having sex, building the tension. Yeh. Foreplay.