Hi and thanks for writing JA
I am sorry that I missed you in chat. It does appear SOLELY FROM WHAT YOU HAVE POSTED that more is going on than just a casual relationship, because you are right, women do not send men pictures of themselves in bikinis and negligees if they are not also sending sexual overtures.
What you do not know at this point is whether or not this is a flirtation which so far is all the proof you have or more. It it was just a flirtation there is nothing really to worry about as many people do this all the time and more so now in the internet generation. Men fantasize about other women all the time which is why publications such as Playboy and Penthouse are so successful.
I would take him at his word. If he says he is not having an affair leave it at that. See how things go in the coming weeks and months. If you find more reason to be suspicious of him confront him and see how he reacts.
I think it would be very much out of line for you to confront the woman in question or speak to her husband and you would be ill thought of by all involved in this matter.
Right now, you have no evidence of an affair just a flirtation so try to take it easy and see how things unfold.
If you have more information you would like to add or more questions to ask , please do.