At least he's using protection I guess...I don't think my ex husband even showed me that much respect and I had the humiliation of going for my first ever STI test at the age of 39.
Op, people can and will advise you many different things but you have to handle this however you feel best for you and your situation/family. I found out about y ex's affair and packed him a bag that same day. When he got home from work I handed him the bag, told him I knew and asked him to leave.
He did, as apart from not being able to resist a bit of female attention, he was and is a decent man in other respects. I didn't gather any documents as they were all in the house he was leaving any way and I just didn't feel the need. It was me who dealt with all of the financial stuff anyway so I don't think he'd have a clue what he was looking for. In fact, 4 years later when we divorced, it was me who gave him most of the documents he needed.
We have a good co-parenting relationship and are planning a university open day trip for our eldest at the moment. Don't get me wrong, I was broken by what he did but I was always thinking of our dc before anything else and by not doing what I really felt like doing at the time (various acts of revenge), I think they have benefitted in the long run, as have I.
But that was me, I probably didn't handle everything as best I could but I did what I felt was right for me and our dc, not what someone else might have done.
Sorry this has happened to you and good luck.