Well, now that it has already happened, I would maybe share my feelings (which you already did) and ask him not to do it again in the future. If he agrees, then you'll also need to let this one slide, so to speak...don't hold it against him anymore.
Also, in the future, your hubby may want to not do one on one lunches with female co-workers for other/professional reasons too--consider how his actions may be interpreted at work by others. An innocent lunch could be misinterpreted by other employees or even the female in question.
I work in an office of nearly all men (sometimes it has been all men and then me). I have occassionally done lunch one on one with my male boss. It is very occassinally and my husband knows my boss socially. He does not have a problem with it. It is usually a reward for good work of some sort. Or to celebrate an "anniversary" like 5 years on the job or whatnot.
I also had a male co-worker ( a really nice guy) that promised me a lunch for some extra work I had done for him, but one of his own rules was not to do a one on one lunch with a female other than his wife, as it just was not in good taste (form his point of view). It was something he had read somewhere along the way in a book about creating a harmoniuos work environment or some such thing. When he bought me my lunch we invited another coworker to join us--problem solved. If no one could go, we would have done it a different day.
Chances are you have nothing to feel worried or jealous about and your husband's explanation was truthful. You might be feeling more vulnerable than usual right now (new baby, hormones, overtiredness, etc.), but that is not to say that your feelings about the situation are invalid. Just one more piece of the puzlze to consider.
Good luck honey--I hope he agrees that it wasn't the best choice and decides not to do it again in the future.
