Those are tough questions. I am not sure (not knowing your husband if this was a test) It sounds to me like he trusts you and thinks it might be nice for you to not be totally alone while your away, but that's just my take on it. I think the danger comes in when there is an attraction and both of you are questioning your marriage. People in basically happy marriages would hopefully not go there. Because you were intimate in the past and we all have our memories some feelings may sneak up on you but I think they will be based on past feelings. Then confusion would set in big time.
As far as being friends with the opposite sex, I think it's possible but it takes a certin personality type. I have men friends and at a weak moment I may find them attractive and may look at them differently. Although these feelings never last. I have had a dream about someone I work with and I wake up thinking about them in a different way and actually look at them differently especially if it was a sexual dream although it is usually just for that day. I think we can talk ourselves into anything especially when we are in an unhealthy place. So....yes you can be just friends with the opposite sex/former boyfriend but it's not possible for some people and even if it is, things change, feelings change, you just have to be aware of it and always act with respect, especially if there is a wife in the picture. I was attracted to my ex but could keep a handle on it. The attraction was based on our past relationship, not on the here and now which is what I finally figured out.
If both your marriages are stable there is nothing to be worried about. Just watch out as some feelings may crop up when you least exect them. All marriages are vulnerable to this.
Kate