Jemma,
There is never a good enough excuse for infidelity. However, there are reasons, some compelling, to understand "why" we, who have been sexually abused, may respond. He may not have stopped for some reason that will only come to light with the right type of counseling.
To use it has an excuse, knowing fair well what he is doing, is neither healthy nor what your marriage (or covenant) is based upon. If he is seeking help, and wants to stop, he will need an accountability partner, someone who he can be held accountable to when he's feeling a need to violate his covenant he made with you.
You'll need to decide to what level of genuineness he has in stopping this behavior. Like an addiction, it may be hard to stop, but it can and should stop. I suspect that being here, he probably does want to stop. He wants to find the answers to why.
As for myself, I had developed a barrier and was hypervigilant that I slept on the edge of our bed, while on my side, gripping the corded roll where the seams of the mattress came to gether. I had even lost interest in intimacy with my bride and never known why. My prostitute was work, I'd put in between 100 and 105 hrs a week. With there only being 168 hrs in a week, there wasn't much time to spend with my wife and kids. I fooled myself into believing I did it for them. But ultimately, I had to deal with the physical manifestations that eventually came, required counseling, job loss and a rebuild of trust.
I know that for me, counseling and talking to first my counselor and with my wife and children, helped me begin the journey towards healing and a better relationship with my wife. For us, having God as the center of our marriage, then each other as second and ourselves as third. If you are willing to watch a movie either with your husband or alone, I would recommend watching "Fireproof". If you desire to make your marriage work, watching this move and getting a copy of "Love Dare" workbook by Kendrick, will give you both the best chance at forgiveness, healing and strengthening your marriage. Most Sincerely, Bill Harry.
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Forgiving does not always mean everything goes back to the way it was. There are still natural consequences for what was done.