I had this issue with my dh a few summers ago, I caught him texting and facebooking a coworker, he said they were just friends. At first I overlooked it after all we've been together for so long. but then she started texting him more often and then I noticed if I asked him a question he would get iffy on me. I took a look at the chat log and while he kept things friendly she was stepping out of line.
I felt her messages were inappropriate to be sent to a married man, things like, what would you liek to see me wear tomorrow? should I wear a low top? ramdom texts winking at him and other stuff, his responses where often what ever, or something of the sort. I think he liked the attention that another woman was giving him. but I being who I am put and end to it right then and there.
I took his phone and texted her and told her flat out I did not appreciate the way she was texting with my dh and then I addressed my dh flat out. told him that I know he is a man, a living breathing man and that I understood that another womans attention after all these years was likely making him feel good, but that I did not appreciate him going behind my back and allowing someone to threaten our relationship. I told him that from here on he could distance himself from her and be professional or he could very well WALK out and not come back. I told him it was hurtful and it made me feel bad and it was hard to trust he wasn't doing more behind my back.
frankly I let him know without losing my cool how I felt about it and I let her know at the same time. I reminded him of everything we had he valued, us, the kids, our way of life, and asked him if he really wanted to throw it all away for someone who is not going to be there for him when he needs someone the most. I then reminded him that I too gave him that sort of attention, that it has always been there. When you tell him first hand how great he is and how much he means to you, another woman saying it to him or flirting with him will mean nothing to him.
My husband did say it was nice to get the attention but that he never thought it was going to affect us. He assured me he loved me, he apologized and then asked her not to text him anymore unless it was work related. She no longer does, she was single and lonely and apparently to her married man were more attractive.
YOu need to have that conversation with your dh and not allow him or her to walk over you. Yes it might lead to a fight or you might be surprised and it will all be calm. Let him see how it makes you feel and be completely honest and stay on topic. tell him flat out, that he needs to respect you and your wishes and that (and this will be hard to admit to him but he will get it immediately) you feel threatened and scared that you will lose him and you just want to see and know that everything is and will be ok. tell him its making your trust in him waver and tell him how that alone makes you feel.
If there is a time to be vulnerable and let it all out on the table this is the time, when it comes to something you really value.
there is no way I will lay down and be walked on without having my opinion heard, and fighting for what is mine and what I want!!!!!!!!!!!!!!!!!!!
stay strong honey, don't let your inner self bring the powerful woman that you are down. 
Ema
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