A few years ago I thought that I had caught something, but it was a false alarm.  I was open about the condition (had small nodules on the boys) with the wife and went to a doc to have everything checked out.  I had not yet hatched up an explanation if tests came back as an STD.  Luckily I didn't have to.

Re: What to tell your Wife?

Perhaps administering first aid. I haven't contracted anything but I had helped two other people administering first aid to a person who fell or jumped from a six story building, and got some of his blood on me. She knew about it, and that was going to be my excuse, but of course it can only be good so long without getting tested.

Tell her you got it from a lot lizard-

Does anyone still have sex with their wife?  If so, send it in to Ripley's.

When I was in the service, I got a case of crabs despite going through a long dry spell at the time.  Go figure.

I'm sure this isn't what you want to hear, but the answer is definitely "It depends".  There are some STI's that it is very common to have for years - with no knowledge, and no symptoms.  Others....not so much.  My best advice is to talk with your doctor or do some good research online about the particular condition and what might be plausible.  You can try telling him or her that you've been married for X number of years, and ask how this would have been possible.  

Of course, I think you're much better of being careful to begin with.

~ Naiya