My earlier post should have said:
"However, all such friendships should have some boundaries, because it would be wholly disingenuous to pretend that it is impossible for those friendships to cross the line, especially when that friendship is new." of course!! 
My post was also written before the other damning evidence arrived by the truckload and so my opinion is definitely that this man is unfaithful and is likely to create opportunities, as well as taking them up, when offered.
I suspect lots of people have learned through bitter experience that someone who was unfaithful before and still believes that behaviour was justified and could be again, will always have a personal permission to be unfaithful. The other massive red flags are the way he has vilified his former partners - something no doubt he'd have no compunction doing to you either.
Other people are diplomatically trying to tell you what is going on, but also bear in mind that he might not have got lucky very often and is regarded as a sleazy pest by others, who sympathise greatly with your position. Your earlier posts for example, suggest that at least one woman has been fobbing him off. I'd also echo the possibility that he has been using prostitutes and if so, his expenditure will provide those clues.
Do you know any of these people well enough to ask them for an honest assessment of your H? People will often be glad of the opportunity to answer a direct question, but might not want to instigate the message.
By the looks of things though, you've got enough to go on with his current behaviour to demonstrate that it is unreasonable, so you don't actually need proof, even though you might want it.