Is sex sinful when you are separated?

Question:

I've looked over all the answers in the divorce section and haven't been able to find an answer to my question. My husband and I are currently separated because he is going through a midlife crisis of sorts. He is unhappy and unfulfilled and is searching for something to fill his void.

I believe God is using this time to lead my husband to Christ (he is not a believer) and to strengthen my personal faith. My desire is to do God's will which I feel is to reconcile with my husband, even though he cheated on me a year ago and it was his decision to leave. The fact that I have forgiven him and still want to make our marriage work is only possible because of divine intervention as far as I’m concerned. Despite the fact that my husband no longer lives here with me and our child, he still comes around quite often, takes care of us financially, always gives me a hug before he leaves, and, yes, we even occasionally have sex.

So my question is: Is it a sin to have sex with my 'estranged' husband? I am still committed to him and as far as I know, he isn't with the other woman anymore. I asked him last night if he was dating anyone, and he said no. I told him that I wasn’t either, and I hoped that even though we are separated, we will remain faithful to each other. We are still legally married and haven't discussed divorce yet. I am just waiting on God to do His work in His time.

Thank you.