I just saw red marks all over both my shoulder after seeing a lady today.  
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That makes me wonder how do savvy gentlemen handle this? Would your SO believe when you said you had to scratch a real bad itch? Or would you refuse to take off your shirt? Anything to make them heal faster or look normal quick?
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Or would you said I am screwed.

If that's impossible, then some uncomplicated remark like:   Oh, some jerk at work was trying to show me how to do self defense moves.

It kind of depends on your own lifestyle, but a golfer could claim he got caught in some bushes looking for a lost ball, A guy who goes to the gym could claim he was in a pick up basketball game and took a tumble, OTOH if you are an office worker who's idea of "outdoor activity" is having his lunch "al fresco" you are probably screwed.  

is Cortisone 10, available over the counter at drug stores, target or walmart.  This comes from a dermatologist friend.  I have been using it for years, although I don't have an SO to worry about, but I just think its bad form to see a provider with marks on your back from yesterday's provider.  I suggest applying every three or four hours until they disappear.  

Ask your SO if there's any of the extra strength Tylenol around (or whatever pain meds you might have used previously)  - we were rough housing in the office and so-and-so got a little carried away - you know so-and-so -  boy are my shoulders sore. . .

Most guys who see the ladies probably aren't seen by their wives naked.

I tee it up waiting for you to rib me just like this LOL.

Hahahahahah Hahahaha haha!

Definitely need to try Cortisone 10. You save me from seeing another provider again. I am thoughtless.

I would be doing damage control instead of asking here if I am married or living together. You didn't hear me panic, right? It seems tough to avoid suspicion when there are marks or unfamiliar scents.

For trying to help. You always have my respect. I tipped my hat to you the first time you answered my question, and I tipped my hat again to you now. A gentleman and a scholar.

It's not that I think I am morally superior to any of you guys who do cheat, it's just that I am too fucking lazy to constantly be lying to the person who is supposed to be the most important person in my life. My solution for not breaking promises is to never (or at least rarely) make them in the first place.  

A single scratch or red mark is one thing to try to explain away as office rough-housing or an accident, but a matching set of four nail marks across your back is another.

- end of story

"Machinery", are you talking about something like the telephone and a calculator, which are both on my desk, or something larger, like a monitor and a printer?  Its a good excuse, but I have to adapt it to my own work environment.  

You can say: I got a free class at a new gym of Jiu-jitsu brasileño, was hard I think I will try once at month. hahaha