Madabout and Hatty, I think you are right here.
I would have hated it if my h had had an affair, because of the emotional involvement.
At first, I thought it was not as bad that he was internet sex chatting.
However, once you get over the shock you start to think it is worse, because if a 50 year old bloke can inquire about the sexual past of women younger than his daughter (and still be shocked at the idea of a man his age doing that to HIS daughters), then what does that tell you about his state of mind, and real attitudes to women? And all the time telling his own wife (complaining about a sexless marriage) that to sleep with anyone else would be a deal breaker.
Despite what he says about his sexual politics.
OP, I think that as soon as you feel less shocked and able to, you should look for more evidence quite thoroughly, and agree get an STD test.
And work out a very careful confrontation strategy, as someone said above, saying less than you know, as you will you see that he will lie.
From my won experience and everything I have read on this board, they almost always lie, and for as long as they can.
And everyone says, sadly, that they would never have suspected it. I wouldnt have, either.